Ensure the export always has an active_object

Some operators, namely the bake operator, require there to be an
active_object. The lightmap operator manages the selection, so this only
fixes Blender whining about the context being incorrect due to a
technicality.

korman/operators/op_export.py

@@ -164,6 +164,11 @@ class _UiHelper:
         self.frame_num = scene.frame_current
         scene.frame_set(scene.frame_start)
         scene.update()
+
+        # Some operators require there be an active_object even though they
+        # don't actually use it...
+        if scene.objects.active is None:
+            scene.objects.active = scene.objects[0]
         return self
 
     def __exit__(self, type, value, traceback):