[layouts] Fix crash when opening layout with dynamic image in html label

src/core/layout/qgslayoutitemlabel.cpp

@@ -157,7 +157,10 @@ void QgsLayoutItemLabel::contentChanged()
       //For very basic html labels with no external assets, the html load will already be
       //complete before we even get a chance to start the QEventLoop. Make sure we check
       //this before starting the loop
-      if ( !mHtmlLoaded )
+
+      // important -- we CAN'T do this when it's a render inside the designer, otherwise the
+      // event loop will mess with the paint event and cause it to be deleted, and BOOM!
+      if ( !mHtmlLoaded && ( !mLayout || !mLayout->renderContext().isPreviewRender() ) )
       {
         //Setup event loop and timeout for rendering html
         QEventLoop loop;
@@ -185,6 +188,8 @@ void QgsLayoutItemLabel::loadingHtmlFinished( bool result )
 {
   Q_UNUSED( result )
   mHtmlLoaded = true;
+  invalidateCache();
+  update();
 }
 
 double QgsLayoutItemLabel::htmlUnitsToLayoutUnits()

src/core/layout/qgslayoutrendercontext.h

@@ -345,6 +345,7 @@ class CORE_EXPORT QgsLayoutRenderContext : public QObject
     friend class TestQgsLayout;
     friend class LayoutContextPreviewSettingRestorer;
     friend class TestQgsLayoutMap;
+    friend class TestQgsLayoutLabel;
 
 };
 

tests/src/core/testqgslayoutlabel.cpp

@@ -299,6 +299,7 @@ void TestQgsLayoutLabel::renderAsHtmlRelative()
 {
   QgsLayout l( QgsProject::instance() );
   l.initializeDefaults();
+  l.renderContext().mIsPreviewRender = false;
   QgsLayoutItemLabel *label = new QgsLayoutItemLabel( &l );
   label->setMargin( 1 );
   l.addLayoutItem( label );