Merge branch 'mkirk/perm-thread' into hotfix/2.20.1

Signal/src/util/UIViewController+Permissions.m

@@ -83,6 +83,7 @@ NS_ASSUME_NONNULL_BEGIN
     };
 
     void (^presentSettingsDialog)(void) = ^(void) {
+        DispatchMainThreadSafe(^{
             UIAlertController *alert = [UIAlertController
                 alertControllerWithTitle:NSLocalizedString(@"MISSING_MEDIA_LIBRARY_PERMISSION_TITLE",
                                              @"Alert title when user has previously denied media library access")
@@ -107,6 +108,7 @@ NS_ASSUME_NONNULL_BEGIN
             [alert addAction:dismissAction];
 
             [self presentViewController:alert animated:YES completion:nil];
+        });
     };
 
     if ([UIApplication sharedApplication].applicationState == UIApplicationStateBackground) {