After calling `systemPreferences.getAccentColor();` app crashes. There is no any error in console, only `Process finished with exit code 0`.  `try/catch` doesn't help.

### Issue Details

* **Electron Version:** 5.0.0
* **Operating System:** macOS 10.14.4
* **Last Known Working Electron version:** unknown

### Expected Behavior
Method `systemPreferences.getAccentColor();` should work as described in docs.

### Actual Behavior
App crashes.

### To Reproduce
Just execute following on macOS 10.14.4 with electron 5.0.0:
```javascript
const {app, systemPreferences} = require('electron');

app.whenReady().then(() => {
    try {
        let accentColor = systemPreferences.getAccentColor();
        console.log(accentColor);
    } catch (e) {
        console.error(e);
    }
});
```
atom/browser/ui/cocoa/NSColor+Hex.h
#ifndef ATOM_BROWSER_UI_COCOA_NSCOLOR_HEX_H_
#define ATOM_BROWSER_UI_COCOA_NSCOLOR_HEX_H_
#import <Cocoa/Cocoa.h>
@interface NSColor (Hex)
+ (NSColor*)colorWithHexColorString:(NSString*)hex;
@end
#endif