Dont try to update an instance that isnt running yet (#15998)

src/lightning_app/components/serve/streamlit.py

@@ -58,6 +58,7 @@ def run(self) -> None:
             ],
             env=env,
         )
+        self._process.wait()
 
     def on_exit(self) -> None:
         if self._process is not None:

src/lightning_app/core/app.py

@@ -526,14 +526,11 @@ def _update_layout(self) -> None:
             component._layout = layout
 
     def _update_is_headless(self) -> None:
-        is_headless = _is_headless(self)
+        self.is_headless = _is_headless(self)
 
         # If `is_headless` changed, handle it.
         # This ensures support for apps which dynamically add a UI at runtime.
-        if self.is_headless != is_headless:
-            self.is_headless = is_headless
-
-            _handle_is_headless(self)
+        _handle_is_headless(self)
 
     def _apply_restarting(self) -> bool:
         self._reset_original_state()

src/lightning_app/utilities/app_helpers.py

@@ -19,7 +19,7 @@
 
 import websockets
 from deepdiff import Delta
-from lightning_cloud.openapi import AppinstancesIdBody, Externalv1LightningappInstance
+from lightning_cloud.openapi import AppinstancesIdBody, Externalv1LightningappInstance, V1LightningappInstanceState
 
 import lightning_app
 from lightning_app.utilities.exceptions import LightningAppStateException
@@ -556,7 +556,12 @@ def _handle_is_headless(app: "LightningApp"):
             "App was not found. Please open an issue at https://github.com/lightning-AI/lightning/issues."
         )
 
-    if current_lightningapp_instance.spec.is_headless == app.is_headless:
+    if any(
+        [
+            current_lightningapp_instance.spec.is_headless == app.is_headless,
+            current_lightningapp_instance.status.phase != V1LightningappInstanceState.RUNNING,
+        ]
+    ):
         return
 
     current_lightningapp_instance.spec.is_headless = app.is_headless