fix: crash on invalid displaynames

lib/src/utils/event_localizations.dart

@@ -144,11 +144,13 @@ abstract class EventLocalizations {
           text = i18n.joinedTheChat(targetName);
         }
       } else if (newMembership == 'join') {
-        final newAvatar = event.content['avatar_url'] ?? '';
+        final newAvatar = event.content.tryGet<String>('avatar_url') ?? '';
-        final oldAvatar = event.prevContent?['avatar_url'] ?? '';
+        final oldAvatar = event.prevContent?.tryGet<String>('avatar_url') ?? '';
 
-        final newDisplayname = event.content['displayname'] ?? '';
+        final newDisplayname =
-        final oldDisplayname = event.prevContent?['displayname'] ?? '';
+            event.content.tryGet<String>('displayname') ?? '';
+        final oldDisplayname =
+            event.prevContent?.tryGet<String>('displayname') ?? '';
         final stateKey = event.stateKey;
 
         // Has the user avatar changed?

test/event_test.dart

@@ -368,6 +368,46 @@ void main() {
           'Removed by Example');
       expect(event.isEventTypeKnown, true);
 
+      event = Event.fromJson({
+        'content': {
+          'avatar_url':
+              'mxc://pixelthefox.net/bmGuC44Eeb3BomfkZTP02DVnGaRp4dek',
+          'displayname': [
+            [
+              [[]]
+            ]
+          ],
+          'membership': 'join'
+        },
+        'origin_server_ts': 1636487843183,
+        'room_id': '!watercooler-v9:maunium.net',
+        'sender': '@nyaaori:pixelthefox.net',
+        'state_key': '@nyaaori:pixelthefox.net',
+        'type': 'm.room.member',
+        'unsigned': {
+          'prev_content': {
+            'avatar_url':
+                'mxc://pixelthefox.net/bmGuC44Eeb3BomfkZTP02DVnGaRp4dek',
+            'displayname': 1,
+            'membership': 'join'
+          },
+          'prev_sender': '@nyaaori:pixelthefox.net',
+          'replaces_state': '\$kcqn2k6kXQKOM45t_p8OA03PQRR3KB2N_PN4HUq1GiY'
+        },
+        'event_id': '\$21DJjleMGcviLoT4L9wvxawMlOXSQ9yW6R8mrhlbhfU',
+        'user_id': '@nyaaori:pixelthefox.net',
+        'replaces_state': '\$kcqn2k6kXQKOM45t_p8OA03PQRR3KB2N_PN4HUq1GiY',
+        'prev_content': {
+          'avatar_url':
+              'mxc://pixelthefox.net/bmGuC44Eeb3BomfkZTP02DVnGaRp4dek',
+          'displayname': 1,
+          'membership': 'join'
+        }
+      }, room);
+      expect(
+          event.getLocalizedBody(MatrixDefaultLocalizations()) is String, true);
+      expect(event.isEventTypeKnown, true);
+
       event = Event.fromJson({
         'content': {
           'body': 'Landing',