Merge pull request #627 from GetStream/ref/non-nullable-streams

refactor(llc, ui): Make Streams non-nullable wherever possible.

packages/stream_chat_flutter/lib/src/channel_preview.dart

@@ -132,7 +132,7 @@ class ChannelPreview extends StatelessWidget {
                                   message: lastMessage!,
                                   size: channelPreviewTheme.indicatorIconSize,
                                   isMessageRead: channel.state!.read
-                                          ?.where((element) =>
+                                          .where((element) =>
                                               element.user.id !=
                                               channel
                                                   .client.state.currentUser!.id)

packages/stream_chat_flutter/lib/src/message_list_view.dart

@@ -946,7 +946,7 @@ class _MessageListViewState extends State<MessageListView> {
     }
 
     final channel = streamChannel!.channel;
-    final readList = channel.state?.read?.where((read) {
+    final readList = channel.state?.read.where((read) {
           if (read.user.id == userId) return false;
           return read.lastRead.isAfter(message.createdAt) ||
               read.lastRead.isAtSameMomentAs(message.createdAt);
@@ -1234,8 +1234,7 @@ class _MessageListViewState extends State<MessageListView> {
           MaterialPageRoute(
             builder: (_) => BetterStreamBuilder<Message>(
               stream: streamChannel!.channel.state!.messagesStream.map(
-                  (messages) =>
-                      messages!.firstWhere((m) => m.id == message.id)),
+                  (messages) => messages.firstWhere((m) => m.id == message.id)),
               initialData: message,
               builder: (_, data) => StreamChannel(
                 channel: streamChannel!.channel,

packages/stream_chat_flutter/test/src/channel_preview_test.dart

@@ -21,6 +21,8 @@ void main() {
       when(() => clientState.currentUser).thenReturn(user);
       when(() => clientState.currentUserStream)
           .thenAnswer((_) => Stream.value(user));
+      when(() => channel.lastMessageAtStream)
+          .thenAnswer((_) => Stream.value(lastMessageAt));
       when(() => channel.lastMessageAt).thenReturn(lastMessageAt);
       when(() => channel.state).thenReturn(channelState);
       when(() => channel.client).thenReturn(client);