Fix the error when block has been removed but still left configured in dashboard

Throws error when block has been removed but still left configured in dashboard.

Steps to reproduce:

1. Create a block_view xyz
2. Add the block xyz to dashboard
3. Remove the view

The website encountered an unexpected error. Please try again later.
Error: Call to a member function getPlugin() on null in Drupal\draggable_dashboard\Plugin\Block\DraggableBlock->build() (line 112 of modules/contrib/draggable_dashboard/src/Plugin/Block/DraggableBlock.php).
Drupal\draggable_dashboard\Plugin\Block\DraggableBlock->build() (Line: 203)
Drupal\block\BlockViewBuilder::preRender(Array)
call_user_func('Drupal\block\BlockViewBuilder::preRender', Array) (Line: 378)
Drupal\Core\Render\Renderer->doRender(Array) (Line: 450)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 195)
Drupal\Core\Render\Renderer->render(Array) (Line: 490)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1) (Line: 117)
__TwigTemplate_a00c60f0082b12d22ef77836cd0c1faf8854ffc120fedc9cc891264a2fceb4ef->doDisplay(Array, Array) (Line: 428)
Twig_Template->displayWithErrorHandling(Array, Array) (Line: 399)
Twig_Template->display(Array) (Line: 407)
Twig_Template->render(Array) (Line: 64)
twig_render_template('themes/custom/particle/apps/drupal/templates/layout/page.html.twig', Array) (Line: 384)
Drupal\Core\Theme\ThemeManager->render('page', Array) (Line: 437)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 195)
Drupal\Core\Render\Renderer->render(Array) (Line: 490)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1) (Line: 84)
__TwigTemplate_6d88d7f2894af37516956268666b41309cdd654bb39ad8137d60ec0bba009105->doDisplay(Array, Array) (Line: 428)
Twig_Template->displayWithErrorHandling(Array, Array) (Line: 399)
Twig_Template->display(Array) (Line: 407)
Twig_Template->render(Array) (Line: 64)
twig_render_template('core/themes/stable/templates/layout/html.html.twig', Array) (Line: 384)
Drupal\Core\Theme\ThemeManager->render('html', Array) (Line: 437)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 195)
Drupal\Core\Render\Renderer->render(Array) (Line: 147)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 582)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 148)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object) (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object)
call_user_func(Array, Object, 'kernel.view', Object) (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object) (Line: 156)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 693)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)