Merge pull request #4320 from PrimozGodec/concurrent-fix

Concurrent: Prevent running new task from on_done, on_exception

Orange/widgets/utils/concurrent.py

@@ -552,6 +552,12 @@ def _on_task_done(self, future: Future):
             self.on_exception(ex)
         else:
             self.on_done(future.result())
+        # This assert prevents user to start new task (call start) from either
+        # on_done or on_exception
+        assert self.__task is None, (
+            "Starting new task from "
+            f"{'on_done' if ex is None else 'on_exception'} is forbidden"
+        )
 
 
 class ConcurrentWidgetMixin(ConcurrentMixin):