add comment about the null reference in getNode and putNode

Author: Sowasvonbot

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/object/GetRegisteredClassNode.java

@@ -138,6 +138,9 @@ static Object lookup(Node inliningTarget,
                     ObjectHashMap.GetNode getNode) {
         try {
             // lookup generated classes
+            // For now, we assume that the keys in the InteropGeneratedClassCache and
+            // InteropTypeRegistry do not call back to any python code.
+            // Otherwise, the pattern listed in IndirectCallContext#enter must be used.
             var possiblePythonClass = getNode.execute(null,
                             inliningTarget,
                             PythonContext.get(inliningTarget).interopGeneratedClassCache,