Fixes the issue "Non valid number as font size results in an uncaught exception."

CHANGELOG.md

@@ -49,6 +49,7 @@ Note that this project **does not** adhere to [Semantic Versioning](http://semve
 - We fixed an issue where changing the font size makes the font size field too small. [#7085](https://github.com/JabRef/jabref/issues/7085)
 - We fixed an issue with TexGroups on Linux systems, where the modification of an aux-file did not trigger an auto-update for TexGroups. Furthermore, the detection of file modifications is now more reliable. [#7412](https://github.com/JabRef/jabref/pull/7412)
 - We fixed an issue where the Unicode to Latex formatter produced wrong results for characters with a codepoint higher than Character.MAX_VALUE. [#7387](https://github.com/JabRef/jabref/issues/7387)
+- We fixed an issue where a non valid value as font size results in an uncaught exception. [#7415](https://github.com/JabRef/jabref/issues/7415)
 
 ### Removed
 

src/main/java/org/jabref/gui/preferences/appearance/AppearanceTab.java

@@ -1,12 +1,16 @@
 package org.jabref.gui.preferences.appearance;
 
+import java.util.regex.Pattern;
+
 import javafx.application.Platform;
 import javafx.fxml.FXML;
 import javafx.geometry.Pos;
 import javafx.scene.control.CheckBox;
 import javafx.scene.control.RadioButton;
 import javafx.scene.control.Spinner;
 import javafx.scene.control.TextField;
+import javafx.scene.control.TextFormatter;
+import javafx.util.converter.IntegerStringConverter;
 
 import org.jabref.gui.preferences.AbstractPreferenceTabView;
 import org.jabref.gui.preferences.PreferencesTab;
@@ -27,6 +31,16 @@ public class AppearanceTab extends AbstractPreferenceTabView<AppearanceTabViewMo
 
     private final ControlsFxVisualizer validationVisualizer = new ControlsFxVisualizer();
 
+    // The fontSizeFormatter formats the input given to the fontSize spinner so that non valid values cannot be entered.
+    private TextFormatter<Integer> fontSizeFormatter = new TextFormatter<Integer>(new IntegerStringConverter(), 9,
+            c -> {
+                if (Pattern.matches("\\d*", c.getText())) {
+                    return c;
+                }
+                c.setText("0");
+                return c;
+            });
+
     public AppearanceTab() {
         ViewLoader.view(this)
                   .root(this)
@@ -48,6 +62,7 @@ public void initialize() {
         fontSize.getEditor().setAlignment(Pos.CENTER_RIGHT);
         fontSize.setValueFactory(AppearanceTabViewModel.fontSizeValueFactory);
         fontSize.getEditor().textProperty().bindBidirectional(viewModel.fontSizeProperty());
+        fontSize.getEditor().setTextFormatter(fontSizeFormatter);
 
         themeLight.selectedProperty().bindBidirectional(viewModel.themeLightProperty());
         themeDark.selectedProperty().bindBidirectional(viewModel.themeDarkProperty());