Fix potential undefined property

Not sure the reason, but I got this error:

```
Cannot read property 'id' of undefined
    at tn (SongTitleField.js:35)
    at Ka (react-dom.production.min.js:153)
    at vl (react-dom.production.min.js:261)
    at sc (react-dom.production.min.js:246)
    at lc (react-dom.production.min.js:246)
```

ui/src/common/SongContextMenu.js

@@ -100,8 +100,8 @@ const SongContextMenu = ({
 }
 
 SongContextMenu.propTypes = {
-  resource: PropTypes.string,
-  record: PropTypes.object,
+  resource: PropTypes.string.isRequired,
+  record: PropTypes.object.isRequired,
   onAddToPlaylist: PropTypes.func,
   visible: PropTypes.bool,
   showStar: PropTypes.bool,
@@ -109,6 +109,8 @@ SongContextMenu.propTypes = {
 
 SongContextMenu.defaultProps = {
   onAddToPlaylist: () => {},
+  record: {},
+  resource: 'song',
   visible: true,
   showStar: true,
   addLabel: true,

ui/src/common/SongTitleField.js

@@ -76,4 +76,9 @@ SongTitleField.propTypes = {
   showTrackNumbers: PropTypes.bool,
 }
 
+SongTitleField.defaultProps = {
+  record: {},
+  showTrackNumbers: false,
+}
+
 export default SongTitleField