Added test to check for error messages when using non-existent runnin…

…g components and not suppressing callback exceptions

tests/integration/callbacks/test_basic_callback.py

@@ -827,7 +827,7 @@ def on_click(_):
 
 def test_cbsc020_callback_running_non_existing_component(dash_duo):
     lock = Lock()
-    app = Dash(__name__)
+    app = Dash(__name__, suppress_callback_exceptions=True)
 
     app.layout = html.Div(
         [
@@ -858,3 +858,60 @@ def on_click(_):
         dash_duo.find_element("#start").click()
 
     dash_duo.wait_for_text_to_equal("#output", "done")
+
+
+def test_cbsc021_callback_running_non_existing_component(dash_duo):
+    lock = Lock()
+    app = Dash(__name__)
+
+    app.layout = html.Div(
+        [
+            html.Button("start", id="start"),
+            html.Div(id="output"),
+        ]
+    )
+
+    @app.callback(
+        Output("output", "children"),
+        Input("start", "n_clicks"),
+        running=[
+            [
+                Output("non_existent_component", "children"),
+                html.B("on", id="content"),
+                "off",
+            ]
+        ],
+        prevent_initial_call=True,
+    )
+    def on_click(_):
+        with lock:
+            pass
+        return "done"
+
+    dash_duo.start_server(
+        app,
+        debug=True,
+        use_reloader=False,
+        use_debugger=True,
+        dev_tools_hot_reload=False,
+    )
+    with lock:
+        dash_duo.find_element("#start").click()
+
+    dash_duo.wait_for_text_to_equal("#output", "done")
+    error_title = "ID running component not found in layout"
+    error_message = [
+        "Component defined in running keyword not found in layout.",
+        'Component id: "non_existent_component"',
+        "This ID was used in the callback(s) for Output(s):",
+        "output.children",
+        "You can suppress this exception by setting",
+        "`suppress_callback_exceptions=True`.",
+    ]
+    # The error should show twice, once for trying to set running on and once for
+    # turning it off.
+    dash_duo.wait_for_text_to_equal(dash_duo.devtools_error_count_locator, "2")
+    for error in dash_duo.find_elements(".dash-fe-error__title"):
+        assert error.text == error_title
+    for error_text in dash_duo.find_elements(".dash-backend-error"):
+        assert all(line in error_text for line in error_message)