[Tkinter] Error in Tkinter

[buran]

We cannot really give you advice, because you don't show your full code.
In any case you can refactor your code, so that any calculations are performed after you enter your data (e.g. require user to click a button to perform calculations) or if you don't want to click a button you can handle the error and/or set default value for the Var/Entry (e.g. 1.0, value will depend on your business case) and/or use if to check z value before performing calculations