wxGUI/nviz: fix starting 3D when no map loaded

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@56492 15284696-431f-4ddb-bdfa-cd5b030d7da7

gui/wxpython/nviz/tools.py

@@ -129,6 +129,8 @@ class NvizToolWindow(FN.FlatNotebook):
         """!Set initial raster and vector map"""
         for ltype in ('raster', 'vector', '3d-raster'):
             selectedLayer = self.parent.GetLayerTree().GetSelectedLayer(multi = False, checkedOnly = True)
+            if selectedLayer is None:
+                continue
             selectedLayer = self.parent.GetLayerTree().GetLayerInfo(selectedLayer, key = 'maplayer')
             layers = self.mapWindow.Map.GetListOfLayers(ltype = ltype, active = True)
             if selectedLayer in layers: