Steps to reproduce the issue

Make sure that the Preview Module Positions is set to enabled
Click on the eye to open the preview

Expected result

Admin template opens in a new window with all the positions outlined etc and no errors

Actual result

javascript error in the console

admin-cpanel-default.js?16f20334e6c7311d184d278cc57500f8:61 Uncaught TypeError: Cannot read property 'getBoundingClientRect' of null
    at Object.resizeGridItem (admin-cpanel-default.js?16f20334e6c7311d184d278cc57500f8:61)
    at admin-cpanel-default.js?16f20334e6c7311d184d278cc57500f8:72
    at Array.forEach (<anonymous>)
    at Object.resizeAllGridItems (admin-cpanel-default.js?16f20334e6c7311d184d278cc57500f8:71)
    at initialise (admin-cpanel-default.js?16f20334e6c7311d184d278cc57500f8:77)