Fix pr 13 removed the nil check from AVLTree >> add: entirely

[pankaj-bind]

PR #13 issue

@jordanmontt I have removed the nil check from AVLTree >> add: entirely

Now,
Modify CTAVLTree.class.st

Before

{ #category : 'adding' }
CTAVLTree >> add: newObject [
    newObject ifNil: [ Error signal: 'Cannot add nil to AVLTree' ].
    root := root addChild: newObject.
    ^ newObject
]

After

{ #category : 'adding' }
CTAVLTree >> add: newObject [
    root := root addChild: newObject.
    ^ newObject
]