fix: warn when encountering invalid node tree in dfs

Author: BrendanParmer

ntp_operator.py

@@ -241,6 +241,10 @@ def dfs(nt: NodeTree) -> None:
             Parameters:
             nt (NodeTree): current node tree in the dependency graph
             """
+            if nt is None:
+                self.report({'ERROR'}, "NodeToPython: Found an invalid node tree. "
+                            "Are all data blocks valid?")
+                return
             if nt not in visited:
                 visited.add(nt)
                 for group_node in [node for node in nt.nodes