Fix right clicking a group and choosing "remove selected entries from…

… this group" leads to error when Bibtex source tab is selected (#8821)

* Fix #8012 by adding validation before setting value to field that object does not have

* Update CHANGELOG.md

CHANGELOG.md

@@ -31,6 +31,7 @@ Note that this project **does not** adhere to [Semantic Versioning](http://semve
 - We fixed an issue where deprecated fields tab is shown when the fields don't contain any values. [#8396](https://github.com/JabRef/jabref/issues/8396)
 - We fixed an issue which allow us to select and open identifiers from a popup list in the maintable [#8758](https://github.com/JabRef/jabref/issues/8758), [8802](https://github.com/JabRef/jabref/issues/8802)
 - We fixed an issue where the escape button had no functionality within the "Filter groups" textfield. [koppor#562](https://github.com/koppor/jabref/issues/562)
+- We fixed an issue where right clicking a group and choose "remove selected entries from this group" leads to error when Bibtex source tab is selected. [#8012](https://github.com/JabRef/jabref/issues/8012)
 
 ### Removed
 

src/main/java/org/jabref/gui/entryeditor/SourceTab.java

@@ -306,7 +306,9 @@ private void storeSource(BibEntry outOfFocusEntry, String text) {
                     new FieldWriter(fieldWriterPreferences).write(fieldName, newValue);
 
                     compound.addEdit(new UndoableFieldChange(outOfFocusEntry, fieldName, oldValue, newValue));
-                    outOfFocusEntry.setField(fieldName, newValue);
+                    if (outOfFocusEntry.getField(fieldName).isPresent()) {
+                        outOfFocusEntry.setField(fieldName, newValue);
+                    }
                 }
             }