Fix exception on preview style edit and selection (#8293)

Fixes #8280

CHANGELOG.md

@@ -55,6 +55,7 @@ Note that this project **does not** adhere to [Semantic Versioning](http://semve
 
 ### Fixed
 
+- We fixed an issue where an exception occured when a preview style was edited and afterwards another preview style selected. [#8280](https://github.com/JabRef/jabref/issues/8280)
 - We fixed an issue where the actions to move a file to a directory were incorrectly disabled. [#7908](https://github.com/JabRef/jabref/issues/7908)
 - We fixed an issue where an exception occurred when a linked online file was edited in the entry editor [#8008](https://github.com/JabRef/jabref/issues/8008)
 - We fixed an issue when checking for a new version when JabRef is used behind a corporate proxy. [#7884](https://github.com/JabRef/jabref/issues/7884)

src/main/java/org/jabref/gui/preferences/preview/PreviewTabViewModel.java

@@ -160,7 +160,6 @@ public void setPreviewLayout(PreviewLayout selectedLayout) {
     }
 
     public void refreshPreview() {
-        layoutProperty.setValue(null);
         setPreviewLayout(chosenSelectionModelProperty.getValue().getSelectedItem());
     }
 

src/main/java/org/jabref/gui/preview/PreviewViewer.java

@@ -196,6 +196,10 @@ private static String createJavaScriptRegex(Pattern regex) {
     }
 
     public void setLayout(PreviewLayout newLayout) {
+        // Change listeners might set the layout to null while the update method is executing, therefore we need to prevent this here
+        if (newLayout == null) {
+            return;
+        }
         layout = newLayout;
         update();
     }