I'm getting the same "Cannot read property 'pasteHTML' of null" when clicking on the "quote" button from chrome.
I debugged it a little bit. getRange in forums/themes/default/scripts/editor.js (line 916) returns null.
getRange calls getSelect, and the oSelection object is returned empty (i.e rangeCount == 0) which is why it returns null.
In getSelect itself both bWantText and bWantHTMLText are undefined, so it goes to line 898 ("this.oFrameWindow.getSelection") which returns the empty selection.
At this point, I thought, I didn't select any text, I just clicked the quote button, why should it call getSelection at all?
Unless maybe there's another piece of code that is supposed to select the text before calling this and the problem is that it didn't?
So I tried again, only this time I selected some text before clicking the quote button, but it didn't seem to help, getSelection still returns an empty selection.
I also tried this.oFrameDocument.getSelection (instead of oFrameWindow) but it also returns an empty selection even when text is selected, so not sure what to think.