Allow `pre_export()` to return None.

korman/exporter/convert.py

@@ -449,7 +449,9 @@ class Exporter:
             for mod in bo.plasma_modifiers.modifiers:
                 proc = getattr(mod, "pre_export", None)
                 if proc is not None:
-                    for i in filter(None, proc(self, bo)):
+                    result = proc(self, bo)
+                    if result is not None:
+                        for i in filter(None, result):
                             handle_temporary(i, bo)
 
         for bl_obj in self._objects: