FIXED: combo box crash due to intercell spacing not being set

If a combo box is put using Interface Builder, with the current master, opening it result in a crash:

```
Uncaught TypeError: Cannot read property 'width' of nullVM22916:47 CGSizeEqualToSizeVM23138:278 $CPTableView__setIntercellSpacing_Objective-J.js:8944 objj_msgSendFast1VM23148:269 $CPComboBox__setListDelegate_Objective-J.js:8944 objj_msgSendFast1VM23148:310 $CPComboBox__popUpListObjective-J.js:8936 objj_msgSendFast0VM23148:628 $CPComboBox___popUpButtonWasClickedObjective-J.js:8936 objj_msgSendFast0VM23148:836 $_CPComboBoxPopUpButton__mouseDown_Objective-J.js:8944 objj_msgSendFast1VM22920:97 $CPObject__performSelector_withObject_Objective-J.js:8952 objj_msgSendFast2VM23044:944 $CPWindow__sendEvent_Objective-J.js:8944 objj_msgSendFast1VM23081:316 $CPApplication__sendEvent_Objective-J.js:8944 objj_msgSendFast1VM23056:741 $CPPlatformWindow__mouseEvent_VM23056:185 mouseEventCallback
```

This patch solves this.

A reduction of the problem can be found here: https://www.dropbox.com/s/w361bjku3d6q24a/ComboCrash.zip?dl=0