Fix #281.

The default definition of `pre_export()` (for documentation purposes)
was hiding the real default implementation of `pre_export()`, causing
any modifiers relying on said default implementation to export nothing.

korman/exporter/convert.py

@@ -431,7 +431,9 @@ class Exporter:
 
         def do_pre_export(bo):
             for mod in bo.plasma_modifiers.modifiers:
-                for i in filter(None, mod.pre_export(self, bo)):
+                proc = getattr(mod, "pre_export", None)
+                if proc is not None:
+                    for i in filter(None, proc(self, bo)):
                         handle_temporary(i, bo)
 
         for bl_obj in self._objects:

korman/properties/modifiers/base.py

@@ -80,6 +80,8 @@ class PlasmaModifierProperties(bpy.types.PropertyGroup):
            Drawables that will render in the same pass"""
         return False
 
+    # This is temporarily commented out to prevent MRO failure. Revisit in Python 3.7
+    '''
     def pre_export(self, exporter, bo: bpy.types.Object) -> Generator:
         """This is the first phase of the modifier export; allowing modifiers to create additonal
            objects or logic nodes to be used by the exporter. To do so, overload this method
@@ -87,6 +89,7 @@ class PlasmaModifierProperties(bpy.types.PropertyGroup):
            when the export completes. PRP objects should generally not be exported in this phase.
         """
         yield
+    '''
 
     @property
     def requires_actor(self):