Revert "Fix right clicking a group and choosing "remove selected entr…

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

This reverts commit 7257350.

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

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