Unable to get property 'createRange' of undefined or null reference

The documentation for document.selection says right at the top:

selection is no longer supported. Starting with Internet Explorer 11, use getSelection. For info, see Compatibility changes.

Change document.selection.createRange().text to document.getSelection().

The problem was exactly what I predicted. You are calling createRange() on a null or undefined reference. Specifically, document.selection is undefined. The error message said exactly what was wrong.