NSCell enhancement

[Ludovic Marcotte]

Hi,

While porting GNUMail.app to MacOS-X, I've come across an annoying
problem.

In Cocoa, if we call NSCell: -setStringValue with nil, we get an
assertion.

I think we should modify NSCell: -setStringValue to:

- (void) setStringValue: (NSString*)aString
{
  NSString *string = aString;

  NSAssert(aString != nil, @"Invalid parameter not satisfying: aString !=
nil");

  _cell.type = NSTextCellType;
  ... 
}

In order to make porting applications to MacOS-X easier. It may break
some GNUstep apps but I think developers should update their code to be
sure they don't pass nil objects to this method.

I'll see if we should also 'enhance' some other methods.

Thanks,
        Ludovic

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