Problems with theme on block admin page

I have run into the same problem described here: #281643: Wrong theme shown on Administer Blocks page (site building / blocks)
In my environment I could backtrace the problem to the Ajax Load module calling ctools_modal_add_js() in hook_init() which in turn calls theme() which calls init_theme() which is not good in the given context: #219910: Calling theme function from hook_init() interferes with administration theme

The attached patch moves this call into a page preprocess function which solves the problem for me.