Don't bake shadows for objects with no drawables

I fail to understand why Blender thinks this is legit, but oh well...
Also, as a bonus, we no longer clobber values in the GoodNeighbor tracker.
I wonder how many bugs this will fix?

korman/exporter/etlight.py

@@ -163,7 +163,10 @@ class LightBaker:
         return None
 
     def _has_valid_material(self, bo):
-        for material in bo.data.materials:
+        data = bo.data
+        if data is None or not isinstance(data, bpy.types.Mesh):
+            return False
+        for material in data.materials:
             if material is not None:
                 return True
         return False
@@ -318,11 +321,15 @@ class LightBaker:
             toggle.track(objs, "hide_render", False)
             for i in bpy.data.objects:
                 i.select = i == objs
+                if not self._has_valid_material(i):
+                    toggle.track(i, "hide_render", True)
         else:
             for i in bpy.data.objects:
                 value = i in objs
                 if value:
                     toggle.track(i, "hide_render", False)
+                elif not self._has_valid_material(i):
+                    toggle.track(i, "hide_render", True)
                 i.select = value
 
 @persistent

korman/helpers.py

@@ -24,7 +24,8 @@ class GoodNeighbor:
         return self
 
     def track(self, cls, attr, value):
-        self._tracking[(cls, attr)] = getattr(cls, attr)
+        if (cls, attr) not in self._tracking:
+            self._tracking[(cls, attr)] = getattr(cls, attr)
         setattr(cls, attr, value)
 
     def __exit__(self, type, value, traceback):