Intercept KeyError on navigation straight to the metadata tab without adding a config.

wazp/callbacks/metadata.py

@@ -2,6 +2,7 @@
 import io
 import pathlib as pl
 import re
+import warnings
 
 import dash
 import dash_bootstrap_components as dbc
@@ -219,7 +220,7 @@ def get_callbacks(app: dash.Dash) -> None:
     )
     def create_metadata_table_and_buttons(
         metadata_output_children: list, app_storage: dict
-    ) -> html.Div:
+    ) -> html.Div | None:
         """Generate html component with a table holding the
         metadata per video and with auxiliary buttons for
         common table manipulations.
@@ -241,8 +242,25 @@ def create_metadata_table_and_buttons(
         html.Div
             html component holding the metadata dash_table and
             the auxiliary buttons for common table manipulations
+
+        Warns
+        -----
+        UserWarning
+            If no configuration is found (from navigating directly
+            to the ROI tab, for example).
         """
 
+        try:
+            app_storage["config"]
+        except KeyError:
+            # we've likely navigated to the metadata page without an input
+            # config yaml file... not necessarily an error
+            warnings.warn("Configuration not yet loaded.")
+            return html.Div(
+                children="No configuration loaded. Please add a "
+                "configuration file from the 'Home' page."
+            )
+
         if not metadata_output_children:
             metadata_table = create_metadata_table_component_from_df(
                 utils.df_from_metadata_yaml_files(
@@ -373,6 +391,7 @@ def create_metadata_table_and_buttons(
                     generate_yaml_tooltip,
                 ]
             )
+        return None
 
     @app.callback(
         Output("metadata-table", "data"),