Undefined index: attributes in Drupal\bootstrap\Plugin\Preprocess\Menu

Sorry to not provide detailed “steps to reproduce”, but I’m in the middle of a complicated (for me) site build and got the following:

Notice: Undefined index: attributes in Drupal\bootstrap\Plugin\Preprocess\Menu->convertAttributes() (line 36 of themes\contrib\bootstrap\src\Plugin\Preprocess\Menu.php).
Drupal\bootstrap\Plugin\Preprocess\Menu->convertAttributes(Array) (Line: 23)
Drupal\bootstrap\Plugin\Preprocess\Menu->preprocessVariables(Object) (Line: 53)
Drupal\bootstrap\Plugin\Preprocess\PreprocessBase->preprocess(Array, 'menu', Array) (Line: 1441)
Drupal\bootstrap\Bootstrap::preprocess(Array, 'menu', Array) (Line: 118)
bootstrap_preprocess(Array, 'menu', Array)
call_user_func_array('bootstrap_preprocess', Array) (Line: 287)
Drupal\Core\Theme\ThemeManager->render('menu', Array) (Line: 422)
Drupal\Core\Render\Renderer->doRender(Array) (Line: 435)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 201)
Drupal\Core\Render\Renderer->render(Array) (Line: 94)
template_preprocess_toolbar(Array, 'toolbar', Array)
call_user_func_array('template_preprocess_toolbar', Array) (Line: 287)
Drupal\Core\Theme\ThemeManager->render('toolbar', Array) (Line: 422)
Drupal\Core\Render\Renderer->doRender(Array) (Line: 435)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 201)
Drupal\Core\Render\Renderer->render(Array) (Line: 479)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1) (Line: 117)
__TwigTemplate_5ca2a3e6b6c9c411839d8224654db4c854537f9c6273c49236b17332b4d4a009->doDisplay(Array, Array) (Line: 405)
Twig\Template->displayWithErrorHandling(Array, Array) (Line: 378)
Twig\Template->display(Array) (Line: 390)
Twig\Template->render(Array) (Line: 55)
twig_render_template('themes/contrib/dxpr_theme/templates/html.html.twig', Array) (Line: 384)
Drupal\Core\Theme\ThemeManager->render('html', Array) (Line: 422)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 201)
Drupal\Core\Render\Renderer->render(Array) (Line: 162)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 564)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 163)
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: 142)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object, 'kernel.view') (Line: 163)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 709)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

To make matters worse, I’m actually using DXPR Theme on the frontend which depends on Bootstrap, plus this coincides somewhat with installing Shortcut Menu. The warning repeats 12 times on the pages where it appears, and I see no relation between the pages it appears on and the ones it doesn’t. Happy to help diagnose in any way I can.