wxGUI: add GetLayerTree() to StandaloneGrassInterface

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@58426 15284696-431f-4ddb-bdfa-cd5b030d7da7

gui/wxpython/core/giface.py

@@ -256,6 +256,9 @@ class StandaloneGrassInterface():
     def GetLayerList(self):
         raise NotImplementedError()
 
+    def GetLayerTree(self):
+        return None
+
     def GetMapDisplay(self):
         """!Get current map display.
         """