fix tests

lib/src/channel_preview.dart

@@ -128,7 +128,7 @@ class ChannelPreview extends StatelessWidget {
       stream: channel.state.messagesStream,
       initialData: channel.state.messages,
       builder: (context, snapshot) {
-        final lastMessage = snapshot.data.lastWhere((m) => m.shadowed != true);
+        final lastMessage = snapshot.data?.lastWhere((m) => m.shadowed != true);
         if (lastMessage == null) {
           return SizedBox();
         }

test/src/channel_preview_test.dart

@@ -24,6 +24,11 @@ void main() {
         'name': 'test name',
       });
       when(channelState.unreadCount).thenReturn(1);
+      when(channelState.messages).thenReturn([
+        Message(
+          text: 'hello',
+        )
+      ]);
       when(channelState.lastMessage).thenReturn(Message(
         text: 'hello',
       ));