Finally solved it, this code helped me.
- (NSRect)frameOfOutlineCellAtRow:(NSInteger)rowIndex
{
NSRect superFrame = [super frameOfOutlineCellAtRow:rowIndex];
// Return NSZeroRect if the row is a group row
if ([[self delegate] respondsToSelector:@selector(outlineView:isGroupItem:)]) {
if ([[self delegate] outlineView:self isGroupItem:[self itemAtRow:rowIndex]]) {
return NSZeroRect;
}
}
return superFrame;
}