Issue #14881: Allow normal non-main thread to spawn a dummy process

Fix suggested by Itay Brandes

Lib/multiprocessing/dummy/__init__.py

@@ -70,7 +70,8 @@ class DummyProcess(threading.Thread):
     def start(self):
         assert self._parent is current_process()
         self._start_called = True
-        self._parent._children[self] = None
+        if hasattr(self._parent, '_children'):
+            self._parent._children[self] = None
         threading.Thread.start(self)
 
     @property