Don't try to compose images before they have been initialized

src/core/qgsmaprenderercustompainterjob.cpp

@@ -257,7 +257,10 @@ void QgsMapRendererCustomPainterJob::doRender()
       layerTime.start();
 
       if ( job.img )
+      {
         job.img->fill( 0 );
+        job.imageInitialized = true;
+      }
 
       job.renderer->render();
 

src/core/qgsmaprendererjob.cpp

@@ -462,6 +462,9 @@ QImage QgsMapRendererJob::composeImage( const QgsMapSettings &settings, const La
     if ( job.layer && job.layer->customProperty( QStringLiteral( "rendering/renderAboveLabels" ) ).toBool() )
       continue; // skip layer for now, it will be rendered after labels
 
+    if ( !job.imageInitialized )
+      continue; // img not safe to compose
+
     painter.setCompositionMode( job.blendMode );
     painter.setOpacity( job.opacity );
 
@@ -488,6 +491,9 @@ QImage QgsMapRendererJob::composeImage( const QgsMapSettings &settings, const La
     if ( !job.layer || !job.layer->customProperty( QStringLiteral( "rendering/renderAboveLabels" ) ).toBool() )
       continue;
 
+    if ( !job.imageInitialized )
+      continue; // img not safe to compose
+
     painter.setCompositionMode( job.blendMode );
     painter.setOpacity( job.opacity );
 

src/core/qgsmaprendererjob.h

@@ -46,6 +46,8 @@ struct LayerRenderJob
 {
   QgsRenderContext context;
   QImage *img; // may be null if it is not necessary to draw to separate image (e.g. sequential rendering)
+  //! True when img has been initialized (filled with transparent pixels) and is safe to compose
+  bool imageInitialized = false;
   QgsMapLayerRenderer *renderer; // must be deleted
   QPainter::CompositionMode blendMode;
   double opacity;

src/core/qgsmaprendererparalleljob.cpp

@@ -244,7 +244,10 @@ void QgsMapRendererParallelJob::renderLayerStatic( LayerRenderJob &job )
     return;
 
   if ( job.img )
+  {
     job.img->fill( 0 );
+    job.imageInitialized = true;
+  }
 
   QTime t;
   t.start();