Not working with IMCE

I have been using DHTML Menu for a while without any issues, and recently I installed IMCE. I found that the IMCE File Browser is not working (no icons and no preview of image etc.) unless I disable DHTML Menu.

So, I wondered if the IMCE File Browser Navigation is being considered as if it is a Drupal menu, and DHTML Menu is trying to DHTML enable it.

I have configured DHTML Menu to enable only for Administrator menu and Navigation menu - this doesn't fix the issue.

I am using Firefox so I used Firebug and found the code where the error occurs.

In Firebug console I get:

$(".menu li").css("margin-left") is undefined

Then, it shows the line of code where the problem occurs. It is line 508 in the code segment shown below:

505 // When using LTR, all icons can be shifted as one, as the text width is not relevant.
506 if (settings.nav == 'bullet' && !rtl) {
507 // Shift overlay to the left by the width of the icon and the distance between icon and text.
508 var shift = '-' + (Math.ceil(($('.menu li').css('margin-left').replace('px', ''))) + 16) + 'px';
509 // Shift the overlay using a negative left-hand offset, and the text using a negative right-hand margin.
510 $('.dhtml-menu-icon').css('left', shift).css('margin-right', shift);
511 }

So, does this mean there is something wrong with CSS or is this a bug in DHTLM Menu ? I am trying to understand why DHTML Menu would be attempting to process the menu, which is actually part of IMCE File Browser. It also looks like this problem only occurs if I use 'bullet' mode in DHTML Menu settings.