Some apps are trying to add event handlers to non-existing DOM elements

[porst17]

document.querySelector("#FileDrop") doesn't match any element such that attaching event handlers afterwards failes with

Uncaught TypeError: Cannot read property 'addEventListener' of null
    at app.html?lang=de:657

The files that contain references to fileDrop are:

lalalab-apps$ grep -R "var fileDrop" .
./circular/app.html:      var fileDrop = document.querySelector("#FileDrop")
./circular/app1.html:      var fileDrop = document.querySelector("#FileDrop")
./pentatonic/app.html:      var fileDrop = document.querySelector("#FileDrop")
./pentatonic/PentaOrna.html:      var fileDrop = document.querySelector("#FileDrop")
./pentatonic/PentaMelody.html:      var fileDrop = document.querySelector("#FileDrop")

Maybe there are more that just name the variable differently. I didn't check that.