lightbox: Replace onDismiss with await showErrorDialog

`showDialog` returns a `Future` that resolves when the dialog is
dismissed.

This refactors `showErrorDialog` to stop overriding `onPressed` and
`barrierDismissible`, and make the caller call Navigator.pop one more
time once the dialog is closed.

This is not an nfc because now the user can tap the barrier (darkened
area around the dialog) to close it.

Note that we check for `mounted` again because there has been an
asynchronous gap.

See also:
  #587 (comment)

Signed-off-by: Zixuan James Li <[email protected]>

lib/widgets/dialog.dart

@@ -15,26 +15,23 @@ Widget _dialogActionText(String text) {
   );
 }
 
+/// Displays an [AlertDialog] with a dismiss button.
+///
+/// Returns a [Future] that resolves when the dialog is closed.
 Future<void> showErrorDialog({
   required BuildContext context,
   required String title,
   String? message,
-  VoidCallback? onDismiss,
 }) {
   final zulipLocalizations = ZulipLocalizations.of(context);
   return showDialog(
     context: context,
-    // `showDialog` doesn't take an `onDismiss`, so dismissing via the barrier
-    // always causes the default dismiss behavior of popping just this route.
-    // When we want a non-default `onDismiss`, disable that.
-    // TODO(upstream): add onDismiss to showDialog, passing through to [ModalBarrier.onDismiss]
-    barrierDismissible: onDismiss == null,
     builder: (BuildContext context) => AlertDialog(
       title: Text(title),
       content: message != null ? SingleChildScrollView(child: Text(message)) : null,
       actions: [
         TextButton(
-          onPressed: onDismiss ?? () => Navigator.pop(context),
+          onPressed: () => Navigator.pop(context),
           child: _dialogActionText(zulipLocalizations.errorDialogContinue)),
       ]));
 }

lib/widgets/lightbox.dart

@@ -473,17 +473,15 @@ class _VideoLightboxPageState extends State<VideoLightboxPage> with PerAccountSt
       await _controller!.play();
     } catch (error) { // TODO(log)
       assert(debugLog("VideoPlayerController.initialize failed: $error"));
-      if (mounted) {
-        final zulipLocalizations = ZulipLocalizations.of(context);
-        await showErrorDialog(
-          context: context,
-          title: zulipLocalizations.errorDialogTitle,
-          message: zulipLocalizations.errorVideoPlayerFailed,
-          onDismiss: () {
-            Navigator.pop(context); // Pops the dialog
-            Navigator.pop(context); // Pops the lightbox
-          });
-      }
+      if (!mounted) return;
+      final zulipLocalizations = ZulipLocalizations.of(context);
+      // Wait until the dialog is closed
+      await showErrorDialog(
+        context: context,
+        title: zulipLocalizations.errorDialogTitle,
+        message: zulipLocalizations.errorVideoPlayerFailed);
+      if (!mounted) return;
+      Navigator.pop(context); // Pops the lightbox
     }
   }