It's unfortunate that the LiquidCrystal library does not play by the rules. Every class is supposed to have a no-argument constructor. All the stuff that the class needs to do it's work is supposed to be defined in the begin() method.
However, there is only one LCD attached to the Arduino, presumably. The LCD should be available to the sketch as well as the MenuLCD library. So, creating the instance of the LiquidCrystal class in the MenuLCD class is not the best approach.
Create the LiquidCrystal instance in the sketch, and pass a reference to that instance to the MenuLCD class.