Fix crash trying to paint in a group layer

src/app/ui/editor/standby_state.cpp

@@ -297,7 +297,7 @@ bool StandbyState::onMouseDown(Editor* editor, MouseMessage* msg)
   }
 
   // Start the Tool-Loop
-  if (layer) {
+  if (layer && layer->isImage()) {
     // Disable layer edges to avoid showing the modified cel
     // information by ExpandCelCanvas (i.e. the cel origin is changed
     // to 0,0 coordinate.)