diff --git a/tools/lammps-gui/chartviewer.cpp b/tools/lammps-gui/chartviewer.cpp
index ee29ab9540..c981b2777d 100644
--- a/tools/lammps-gui/chartviewer.cpp
+++ b/tools/lammps-gui/chartviewer.cpp
@@ -124,10 +124,10 @@ void ChartWindow::add_data(int step, double data, int index)
 
 void ChartWindow::quit()
 {
-    LammpsGui *main;
+    LammpsGui *main = nullptr;
     for (QWidget *widget : QApplication::topLevelWidgets())
         if (widget->objectName() == "LammpsGui") main = dynamic_cast<LammpsGui *>(widget);
-    main->quit();
+    if (main) main->quit();
 }
 
 void ChartWindow::reset_zoom()
@@ -138,10 +138,10 @@ void ChartWindow::reset_zoom()
 
 void ChartWindow::stop_run()
 {
-    LammpsGui *main;
+    LammpsGui *main = nullptr;
     for (QWidget *widget : QApplication::topLevelWidgets())
         if (widget->objectName() == "LammpsGui") main = dynamic_cast<LammpsGui *>(widget);
-    main->stop_run();
+    if (main) main->stop_run();
 }
 
 void ChartWindow::saveAs()
diff --git a/tools/lammps-gui/imageviewer.cpp b/tools/lammps-gui/imageviewer.cpp
index 7406f1860a..00b08f3f47 100644
--- a/tools/lammps-gui/imageviewer.cpp
+++ b/tools/lammps-gui/imageviewer.cpp
@@ -533,10 +533,10 @@ void ImageViewer::copy() {}
 
 void ImageViewer::quit()
 {
-    LammpsGui *main;
+    LammpsGui *main = nullptr;
     for (QWidget *widget : QApplication::topLevelWidgets())
         if (widget->objectName() == "LammpsGui") main = dynamic_cast<LammpsGui *>(widget);
-    main->quit();
+    if (main) main->quit();
 }
 
 void ImageViewer::saveFile(const QString &fileName)
diff --git a/tools/lammps-gui/logwindow.cpp b/tools/lammps-gui/logwindow.cpp
index 374c4c89b1..73ec81d06c 100644
--- a/tools/lammps-gui/logwindow.cpp
+++ b/tools/lammps-gui/logwindow.cpp
@@ -57,18 +57,18 @@ void LogWindow::closeEvent(QCloseEvent *event)
 
 void LogWindow::quit()
 {
-    LammpsGui *main;
+    LammpsGui *main = nullptr;
     for (QWidget *widget : QApplication::topLevelWidgets())
         if (widget->objectName() == "LammpsGui") main = dynamic_cast<LammpsGui *>(widget);
-    main->quit();
+    if (main) main->quit();
 }
 
 void LogWindow::stop_run()
 {
-    LammpsGui *main;
+    LammpsGui *main = nullptr;
     for (QWidget *widget : QApplication::topLevelWidgets())
         if (widget->objectName() == "LammpsGui") main = dynamic_cast<LammpsGui *>(widget);
-    main->stop_run();
+    if (main) main->stop_run();
 }
 
 void LogWindow::save_as()
diff --git a/tools/lammps-gui/preferences.cpp b/tools/lammps-gui/preferences.cpp
index c760e6610b..f7fe9ec025 100644
--- a/tools/lammps-gui/preferences.cpp
+++ b/tools/lammps-gui/preferences.cpp
@@ -286,12 +286,12 @@ GeneralTab::GeneralTab(QSettings *_settings, LammpsWrapper *_lammps, QWidget *pa
 
 void GeneralTab::updatefonts(const QFont &all, const QFont &text)
 {
-    LammpsGui *main;
+    LammpsGui *main = nullptr;
     for (QWidget *widget : QApplication::topLevelWidgets())
         if (widget->objectName() == "LammpsGui") main = dynamic_cast<LammpsGui *>(widget);
 
     QApplication::setFont(all);
-    main->ui->textEdit->document()->setDefaultFont(text);
+    if (main) main->ui->textEdit->document()->setDefaultFont(text);
 }
 
 void GeneralTab::newallfont()
diff --git a/tools/lammps-gui/slideshow.cpp b/tools/lammps-gui/slideshow.cpp
index 92eca9a530..140c703ca3 100644
--- a/tools/lammps-gui/slideshow.cpp
+++ b/tools/lammps-gui/slideshow.cpp
@@ -199,18 +199,18 @@ void SlideShow::loadImage(int idx)
 
 void SlideShow::quit()
 {
-    LammpsGui *main;
+    LammpsGui *main = nullptr;
     for (QWidget *widget : QApplication::topLevelWidgets())
         if (widget->objectName() == "LammpsGui") main = dynamic_cast<LammpsGui *>(widget);
-    main->quit();
+    if (main) main->quit();
 }
 
 void SlideShow::stop_run()
 {
-    LammpsGui *main;
+    LammpsGui *main = nullptr;
     for (QWidget *widget : QApplication::topLevelWidgets())
         if (widget->objectName() == "LammpsGui") main = dynamic_cast<LammpsGui *>(widget);
-    main->stop_run();
+    if (main) main->stop_run();
 }
 
 void SlideShow::movie()