Add error when returning value in no output callbacks.

Author: T4rk1n

dash/_callback.py

@@ -10,6 +10,7 @@
     Output,
 )
 from .exceptions import (
+    InvalidCallbackReturnValue,
     PreventUpdate,
     WildcardInLongCallback,
     MissingLongCallbackManagerError,
@@ -467,6 +468,10 @@ def add_context(*args, **kwargs):
                 raise PreventUpdate
 
             if no_output:
+                if output_value is not None:
+                    raise InvalidCallbackReturnValue(
+                        f"No output callback received return value: {output_value}"
+                    )
                 output_value = []
                 flat_output_values = []
             elif not multi: