Why Error? The operation + cannot accept the arguments

I want to make a Caesar cipher programming application with an app inventer.

However, an error appeared as follows.

From here I don't know what to do. Please help me. I'm desperate for your help.

before doing any calculation, validate your data
in your case make sure, that Key_txt.Text is not empty
do not initialize global variable KEY like this, initialize it before calculating and check, if a value has been entered

if Key_txt.Text is empty
then display a notifier: please enter a value
else do the calculation


Like @Taifun said.
See the circled init.

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