add basic testing for channel_preview widget

lib/src/channel_image.dart

@@ -71,7 +71,7 @@ class ChannelImage extends StatelessWidget {
           String image;
           if (snapshot.data?.containsKey('image') == true) {
             image = snapshot.data['image'];
-          } else if (channel.state.members.length == 2) {
+          } else if (channel.state.members?.length == 2) {
             final otherMember = channel.state.members
                 .firstWhere((member) => member.user.id != client.user.id);
             image = otherMember.user.extraData['image'];

lib/src/typing_indicator.dart

@@ -34,7 +34,7 @@ class TypingIndicator extends StatelessWidget {
       builder: (context, snapshot) {
         return AnimatedSwitcher(
           duration: Duration(milliseconds: 300),
-          child: snapshot.data.isNotEmpty
+          child: snapshot.data?.isNotEmpty == true
               ? Align(
                   key: Key('typings'),
                   alignment: alignment,