fix crash when setting title while to content view is set

AppKit/CPBox.j

@@ -352,6 +352,7 @@ CPBelowBottom = 6;
             [_titleView setAutoresizingMask:CPViewMinYMargin];
             break;
     }
+
     [self sizeToFit];
     [self setNeedsDisplay:YES];
 }
@@ -361,6 +362,9 @@ CPBelowBottom = 6;
     var contentFrame = [_contentView frame],
         offset = [self _titleHeightOffset];
 
+    if (!contentFrame)
+        return;
+
     [_contentView setAutoresizingMask:CPViewNotSizable];
     [self setFrameSize:CGSizeMake(contentFrame.size.width + _contentMargin.width * 2,
                                   contentFrame.size.height + _contentMargin.height * 2 + offset[0])];