Fix for node menu: Show the group input/output nodes in the Input/Output categories respectively, so they can be added with the usual UI in case the user deletes them. These nodes are polled out for

non-group trees (node trees not in the bpy.data.node_groups collection) to avoid confusion. For that purpose a new optional poll function argument has been added to NodeItem, which allows selectively
polling individual items in an otherwise static list.

release/scripts/modules/nodeitems_utils.py

@@ -36,13 +36,18 @@ class NodeCategory():
         elif callable(items):
             self.items = items
         else:
-            self.items = lambda context: items
+            def items_gen(context):
+                for item in items:
+                    if item.poll is None or item.poll(context):
+                        yield item
+            self.items = items_gen
 
 class NodeItem():
-    def __init__(self, nodetype, label=None, settings={}):
+    def __init__(self, nodetype, label=None, settings={}, poll=None):
         self.nodetype = nodetype
         self._label = label
         self.settings = settings
+        self.poll = poll
 
     @property
     def label(self):