Fix Coverity issue #507423 Uninitialize pointer field

src/common/Export.cpp

@@ -209,7 +209,7 @@ void Export::open_export_window(QWidget *parent, RenderLayout *render_layout) {
 
 void Export::open_export_variable_window(const std::map<std::string, Variable *> &variable_list, const std::string &export_filename) {
     Ui_counter_choice_box export_variable_window;
-    QDialog dialog_box;
+    QDialog dialog_box(nullptr);
     export_variable_window.setupUi(&dialog_box);
 
     for (std::map<std::string, Variable *>::const_iterator it = variable_list.begin();