Fix crash when receiving whisper (#2298)

PR #2284 introduced this bug: whispers aren't linked to a twitch channel
but we're storing user colors in a twitch channel. So, dereferencing
a nullptr. Not good.

src/common/ChannelChatters.cpp

@@ -72,7 +72,7 @@ void ChannelChatters::setChatters(UsernameSet &&set)
 
 const QColor ChannelChatters::getUserColor(const QString &user)
 {
-    const auto chatterColors = this->chatterColors_.access();
+    const auto chatterColors = this->chatterColors_.accessConst();
 
     const auto search = chatterColors->find(user.toLower());
     if (search == chatterColors->end())

src/providers/twitch/TwitchMessageBuilder.cpp

@@ -600,7 +600,10 @@ void TwitchMessageBuilder::parseUsername()
     //    }
 
     this->message().loginName = this->userName;
-    this->twitchChannel->setUserColor(this->userName, this->usernameColor_);
+    if (this->twitchChannel != nullptr)
+    {
+        this->twitchChannel->setUserColor(this->userName, this->usernameColor_);
+    }
 
     // Update current user color if this is our message
     auto currentUser = getApp()->accounts->twitch.getCurrent();