dont' crash if we don't have a launcherTasksModel

If we don't have a launcher tasks model it's obvious no item can match
an item in the launcher tasks model.

libtaskmanager/tasksmodel.cpp

@@ -921,6 +921,9 @@ QVariant TasksModel::data(const QModelIndex &proxyIndex, int role) const
         if (proxyIndex.data(AbstractTasksModel::IsLauncher).toBool()) {
             return true;
         } else {
+            if (!d->launcherTasksModel) {
+                return false;
+            }
             for (int i = 0; i < d->launcherTasksModel->rowCount(); ++i) {
                 const QModelIndex &launcherIndex = d->launcherTasksModel->index(i, 0);