handle bad layers when custom layer order is set (fixes #12733)

(cherry picked from commit 78c60a2)

Author: jef-n

src/gui/layertree/qgslayertreemapcanvasbridge.cpp

@@ -86,6 +86,17 @@ void QgsLayerTreeMapCanvasBridge::setCustomLayerOrder( const QStringList& order
   QStringList sortedNewOrder = order;
   qSort( defOrder );
   qSort( sortedNewOrder );
+
+  if ( defOrder.size() < sortedNewOrder.size() )
+  {
+    // some bad layers in the list?
+    for ( int i = 0; i < sortedNewOrder.size(); i++ )
+    {
+      if ( i >= defOrder.size() || defOrder[i] != sortedNewOrder[i] )
+        sortedNewOrder.removeAt( i-- );
+    }
+  }
+
   if ( defOrder != sortedNewOrder )
     return; // must be permutation of the default order