Widget makes the app crash if used without a mask

Hi @santalu! 

The widget makes the app crash if it is  used without a mask. It is becasue a null listener is added on the  overriden function onAttachedToWindow

I have replaced line 65 of MaskEditText.kt with the following validation to fix the issue

if(maskChangedListener != null) {
            addTextChangedListener(maskChangedListener)
}

Good library!! 

Thank you!

Cesar.-
