QmlDesigner: Fix deletion of connections

Since we have multiple instances of this class we cannot use
a static variable. We have to use a member instead.

Task-number: QDS-14451
Change-Id: Ia85c145fb0b8a491132ca270cb62fb75809a03a0
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
(cherry picked from commit 8800e041d2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/qmldesigner/components/toolbar/toolbarbackend.cpp

@@ -393,13 +393,16 @@ ToolBarBackend::ToolBarBackend(QObject *parent)
             &ToolBarBackend::documentIndexChanged);
 
     connect(Core::EditorManager::instance(), &Core::EditorManager::currentEditorChanged, this, [this] {
-        static QMetaObject::Connection lastConnection;
-        disconnect(lastConnection);
+        disconnect(m_documentConnection);
 
         if (auto textDocument = qobject_cast<TextEditor::TextDocument *>(
                 Core::EditorManager::currentDocument())) {
-            lastConnection = connect(textDocument->document(), &QTextDocument::modificationChanged,
-                    this, &ToolBarBackend::isDocumentDirtyChanged);
+            m_documentConnection = connect(textDocument->document(),
+                                           &QTextDocument::modificationChanged,
+                                           this,
+
+                                           &ToolBarBackend::isDocumentDirtyChanged);
+
             emit isDocumentDirtyChanged();
         }
     });

src/plugins/qmldesigner/components/toolbar/toolbarbackend.h

@@ -235,6 +235,7 @@ private:
 
     QStringList m_openDocuments;
     QMetaObject::Connection m_kitConnection;
+    QMetaObject::Connection m_documentConnection;
 };
 
 } // namespace QmlDesigner