Hi Wasay, this comes a little late but for the sake of having this question answered:
Qlik Sense desktop is a web application that runs inside a custom browser on your machine, so what you see on your screen is the user interface for the qlik engine which runs in the background. What runs in the background is the "server", and I guess that the error displayed is a generic error.
Look at this link on how to troubleshoot your app. Open your app and a sheet, hold Ctrl + Shift keys together and right click on the canvas and select "Show Dev tools".
It's difficult to tell anything from your minimal example, but it looks like the variable name isn't defined anywhere, I guess that's why the app crashes. The console in the dev tool tab will tell you right away what's the problem.