Strict warning: call_user_func_array() expects parameter 1 and Warning: Illegal offset type in unset

PHP: 5.6.33 (XAMPP Windows)
Drupal: 8.5.3

Feeds: 8.x-3.0-alpha1
Tamper: 8.x-1.x-dev updated 25 May 2018
Feeds Tamper: 8.x-2.x-dev updated 25 May 2018

When creating a tamper mapping I get the following message in the protocol:

Strict warning: call_user_func_array() expects parameter 1 to be a valid callback, non-static method Drupal\feeds_tamper\Form\TamperFormBase::getPluginForm() should not be called statically in Drupal\Core\Form\FormAjaxResponseBuilder->buildResponse() (Zeile 69 in C:\xampp\htdocs\...\core\lib\Drupal\Core\Form\FormAjaxResponseBuilder.php) #0 C:\xampp\htdocs\...\core\includes\bootstrap.inc(582): _drupal_error_handler_real(2048, 'call_user_func_...', 'C:\\xampp\\htdocs...', 69, Array) #1 [internal function]: _drupal_error_handler(2048, 'call_user_func_...', 'C:\\xampp\\htdocs...', 69, Array) #2 C:\xampp\htdocs\...\core\lib\Drupal\Core\Form\FormAjaxResponseBuilder.php(69): call_user_func_array('Drupal\\feeds_ta...', Array) #3 C:\xampp\htdocs\...\core\lib\Drupal\Core\Form\EventSubscriber\FormAjaxSubscriber.php(98): Drupal\Core\Form\FormAjaxResponseBuilder->buildResponse(Object(Symfony\Component\HttpFoundation\Request), Array, Object(Drupal\Core\Form\FormState), Array) #4 [internal function]: Drupal\Core\Form\EventSubscriber\FormAjaxSubscriber->onException(Object(Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent), 'kernel.exceptio...', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher)) #5 C:\xampp\htdocs\...\core\lib\Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher.php(111): call_user_func(Array, Object(Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent), 'kernel.exceptio...', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher)) #6 C:\xampp\htdocs\...\vendor\symfony\http-kernel\HttpKernel.php(228): Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.exceptio...', Object(Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent)) #7 C:\xampp\htdocs\...\vendor\symfony\http-kernel\HttpKernel.php(79): Symfony\Component\HttpKernel\HttpKernel->handleException(Object(Drupal\Core\Form\FormAjaxException), Object(Symfony\Component\HttpFoundation\Request), 1) #8 C:\xampp\htdocs\...\core\lib\Drupal\Core\StackMiddleware\Session.php(57): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #9 C:\xampp\htdocs\...\core\lib\Drupal\Core\StackMiddleware\KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #10 C:\xampp\htdocs\...\core\modules\page_cache\src\StackMiddleware\PageCache.php(99): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #11 C:\xampp\htdocs\...\core\modules\page_cache\src\StackMiddleware\PageCache.php(78): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true) #12 C:\xampp\htdocs\...\core\lib\Drupal\Core\StackMiddleware\ReverseProxyMiddleware.php(47): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #13 C:\xampp\htdocs\...\core\lib\Drupal\Core\StackMiddleware\NegotiationMiddleware.php(50): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #14 C:\xampp\htdocs\...\vendor\stack\builder\src\Stack\StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #15 C:\xampp\htdocs\...\core\lib\Drupal\Core\DrupalKernel.php(664): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #16 C:\xampp\htdocs\...\index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request)) #17 {main}.

When deleting the tamper mapping I get the message

Warning: Illegal offset type in unset in Drupal\Core\Plugin\DefaultLazyPluginCollection->removeInstanceId() (Zeile 185 in C:\xampp\htdocs\...\core\lib\Drupal\Core\Plugin\DefaultLazyPluginCollection.php) #0 C:\xampp\htdocs\...\core\includes\bootstrap.inc(582): _drupal_error_handler_real(2, 'Illegal offset ...', 'C:\\xampp\\htdocs...', 185, Array) #1 C:\xampp\htdocs\...\core\lib\Drupal\Core\Plugin\DefaultLazyPluginCollection.php(185): _drupal_error_handler(2, 'Illegal offset ...', 'C:\\xampp\\htdocs...', 185, Array) #2 C:\xampp\htdocs\...\modules\feeds_tamper\src\FeedTypeTamperMeta.php(141): Drupal\Core\Plugin\DefaultLazyPluginCollection->removeInstanceId(Object(Drupal\tamper\Plugin\Tamper\Trim)) #3 C:\xampp\htdocs\...\modules\feeds_tamper\src\Form\TamperDeleteForm.php(90): Drupal\feeds_tamper\FeedTypeTamperMeta->removeTamper(Object(Drupal\tamper\Plugin\Tamper\Trim)) #4 [internal function]: Drupal\feeds_tamper\Form\TamperDeleteForm->submitForm(Array, Object(Drupal\Core\Form\FormState)) #5 C:\xampp\htdocs\...\core\lib\Drupal\Core\Form\FormSubmitter.php(111): call_user_func_array(Array, Array) #6 C:\xampp\htdocs\...\core\lib\Drupal\Core\Form\FormSubmitter.php(51): Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object(Drupal\Core\Form\FormState)) #7 C:\xampp\htdocs\...\core\lib\Drupal\Core\Form\FormBuilder.php(585): Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object(Drupal\Core\Form\FormState)) #8 C:\xampp\htdocs\...\core\lib\Drupal\Core\Form\FormBuilder.php(314): Drupal\Core\Form\FormBuilder->processForm('feeds_tamper_de...', Array, Object(Drupal\Core\Form\FormState)) #9 C:\xampp\htdocs\...\core\lib\Drupal\Core\Controller\FormController.php(74): Drupal\Core\Form\FormBuilder->buildForm(Object(Drupal\feeds_tamper\Form\TamperDeleteForm), Object(Drupal\Core\Form\FormState)) #10 [internal function]: Drupal\Core\Controller\FormController->getContentResult(Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\RouteMatch)) #11 C:\xampp\htdocs\...\core\lib\Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array(Array, Array) #12 C:\xampp\htdocs\...\core\lib\Drupal\Core\Render\Renderer.php(582): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() #13 C:\xampp\htdocs\...\core\lib\Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber.php(124): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure)) #14 C:\xampp\htdocs\...\core\lib\Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) #15 [internal function]: Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() #16 C:\xampp\htdocs\...\vendor\symfony\http-kernel\HttpKernel.php(151): call_user_func_array(Object(Closure), Array) #17 C:\xampp\htdocs\...\vendor\symfony\http-kernel\HttpKernel.php(68): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1) #18 C:\xampp\htdocs\...\core\lib\Drupal\Core\StackMiddleware\Session.php(57): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #19 C:\xampp\htdocs\...\core\lib\Drupal\Core\StackMiddleware\KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #20 C:\xampp\htdocs\...\core\modules\page_cache\src\StackMiddleware\PageCache.php(99): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #21 C:\xampp\htdocs\...\core\modules\page_cache\src\StackMiddleware\PageCache.php(78): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true) #22 C:\xampp\htdocs\...\core\lib\Drupal\Core\StackMiddleware\ReverseProxyMiddleware.php(47): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #23 C:\xampp\htdocs\...\core\lib\Drupal\Core\StackMiddleware\NegotiationMiddleware.php(50): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #24 C:\xampp\htdocs\...\vendor\stack\builder\src\Stack\StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #25 C:\xampp\htdocs\...\core\lib\Drupal\Core\DrupalKernel.php(664): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #26 C:\xampp\htdocs\...\index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request)) #27 {main}.