Hello Matthias,
Based on the functionality you mentioned, I would recommend you to check if the document is ready to load with relevant dependencies.
The code example:
function GridRowEnterEditMode (src, validationGroup) {
if (document.readyState != "complete")
{
//Add some delay if document.readyState is not completed
window.setTimeout("GridRowEnterEditMode ()",500); }
else { //Code logic goes here } }
I hope this helps.