Fixes for logging out the active connection

- Don't crash
- Fall back to WelcomePage

src/app/qml/Main.qml

@@ -267,6 +267,17 @@ Kirigami.ApplicationWindow {
         }
     }
 
+    Connections {
+        target: root.connection
+        function onLoggedOut(): void {
+            root.pageStack.clear();
+            let page = root.pageStack.push(Qt.createComponent('org.kde.neochat.login', 'WelcomePage'), {
+                showExisting: true,
+            }) as WelcomePage;
+            page.connectionChosen.connect(() => root.load())
+        }
+    }
+
     Connections {
         target: AccountRegistry
         function onRowsRemoved() {

src/app/roommanager.cpp

@@ -478,9 +478,8 @@ void RoomManager::setConnection(NeoChatConnection *connection)
 
     m_connection = connection;
 
-    m_lastRoomConfig = m_config->group(m_connection->userId()).group(u"LastOpenRoom"_s);
-
     if (m_connection != nullptr) {
+        m_lastRoomConfig = m_config->group(m_connection->userId()).group(u"LastOpenRoom"_s);
         connect(m_connection, &NeoChatConnection::showMessage, this, &RoomManager::showMessage);
         connect(m_connection, &NeoChatConnection::createdRoom, this, [this](Quotient::Room *room) {
             resolveResource(room->id());