Undefined variable warning when adding an Instagram block

Problem/Motivation

When adding an Instagram block to a node and publishing the node, I received the following error:

Notice: Undefined variable: items in Drupal\socialfeed\Plugin\Block\InstagramPostBlock->build() (line 168 of modules/contrib/socialfeed/src/Plugin/Block/InstagramPostBlock.php).
Drupal\socialfeed\Plugin\Block\InstagramPostBlock->build() (Line: 135)
Drupal\panels\Plugin\DisplayBuilder\StandardDisplayBuilder->buildRegions(Array, Array) (Line: 176)
Drupal\panels\Plugin\DisplayBuilder\StandardDisplayBuilder->build(Object) (Line: 219)
Drupal\panels_ipe\Plugin\DisplayBuilder\InPlaceEditorDisplayBuilder->build(Object) (Line: 329)
Drupal\panels\Plugin\DisplayVariant\PanelsDisplayVariant->build() (Line: 396)
Drupal\panelizer\PanelizerEntityViewBuilder->buildPanelized(Object, Object, 'full', NULL) (Line: 356)
Drupal\panelizer\PanelizerEntityViewBuilder->buildMultiplePanelized(Array, Array, 'full', NULL) (Line: 274)
Drupal\panelizer\PanelizerEntityViewBuilder->view(Object, 'full') (Line: 97)
Drupal\Core\Entity\Controller\EntityViewController->view(Object, 'full', NULL) (Line: 55)
Drupal\node\Controller\NodeViewController->view(Object, 'full', NULL)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 582)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 151)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 67)
Drupal\simple_oauth\HttpMiddleware\BasicAuthSwap->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: 54)
Drupal\jsonapi\StackMiddleware\FormatSetter->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: 666)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)


Warning: Invalid argument supplied for foreach() in template_preprocess_item_list() (line 1080 of core/includes/theme.inc).
template_preprocess_item_list(Array, 'item_list', Array) (Line: 287)
Drupal\Core\Theme\ThemeManager->render('item_list', Array) (Line: 437)
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: 83)
__TwigTemplate_0c28051e80d90fd28f5db17303ed2787e91aea7f96ab13d0fe8970bb34353ca6->block_content(Array, Array) (Line: 215)
Twig_Template->displayBlock('content', Array, Array) (Line: 72)
__TwigTemplate_0c28051e80d90fd28f5db17303ed2787e91aea7f96ab13d0fe8970bb34353ca6->doDisplay(Array, Array) (Line: 432)
Twig_Template->displayWithErrorHandling(Array, Array) (Line: 403)
Twig_Template->display(Array) (Line: 411)
Twig_Template->render(Array) (Line: 64)
twig_render_template('themes/custom/bs4/templates/blocks/block.html.twig', Array) (Line: 384)
Drupal\Core\Theme\ThemeManager->render('block', Array) (Line: 437)
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: 58)
__TwigTemplate_80c88d9b0db86f0e1be77ed54132e78661119c1f60d661ece6bc306d37098d5f->doDisplay(Array, Array) (Line: 432)
Twig_Template->displayWithErrorHandling(Array, Array) (Line: 403)
Twig_Template->display(Array) (Line: 411)
Twig_Template->render(Array) (Line: 64)
twig_render_template('themes/custom/bs4/layouts/layout--onecol.html.twig', Array) (Line: 384)
Drupal\Core\Theme\ThemeManager->render('layout__onecol', 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: 92)
__TwigTemplate_593995916bb379c76ab4921d9bb62c05ef9b0a91f048a86c9efdd38d9e83a551->doDisplay(Array, Array) (Line: 432)
Twig_Template->displayWithErrorHandling(Array, Array) (Line: 403)
Twig_Template->display(Array) (Line: 411)
Twig_Template->render(Array) (Line: 64)
twig_render_template('modules/contrib/panelizer/templates/panelizer-view-mode.html.twig', Array) (Line: 384)
Drupal\Core\Theme\ThemeManager->render('panelizer_view_mode', Array) (Line: 437)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 195)
Drupal\Core\Render\Renderer->render(Array, ) (Line: 226)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 582)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 227)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object, Object) (Line: 117)
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: 67)
Drupal\simple_oauth\HttpMiddleware\BasicAuthSwap->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: 54)
Drupal\jsonapi\StackMiddleware\FormatSetter->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: 666)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Proposed resolution

Define the items variable that is used on line 168 of src/Plugin/Block/InstagramPostBlock.php