Fix JabRef#8012 by adding validation before setting value to field th…

…at object does not have

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);
+                    }
                 }
             }