wxGUI/lmgr: fix add group layer during vector map editing (#1109)

gui/wxpython/lmgr/layertree.py

@@ -1783,7 +1783,7 @@ class LayerTree(treemixin.DragAndDrop, CT.CustomTreeCtrl):
 
             if digitToolbar.GetLayer() == mapLayer:
                 self._setGradient('vdigit')
-            elif bgmap == mapLayer.GetName():
+            elif mapLayer and bgmap == mapLayer.GetName():
                 self._setGradient('bgmap')
             else:
                 self._setGradient()