lightbox: Replace onDismiss with await showErrorDialog

The `showDialog` function returns a future that resolves when the
dialog is dismissed, and showErrorDialog returns that future.

The one caller of showErrorDialog that was passing an onDismiss
callback can therefore await that future instead.  Do that, and
eliminate the parameter.

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

Discussion from when this parameter was introduced:
  #587 (comment)

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

Author: PIG208

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

@@ -474,17 +474,15 @@ 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
     }
   }