If you are having the same issue I was with explorers returning from wasteland, then freezing the whole UI when you tried to collect, you need to workaround the bug until it gets fixed. Here are some that might work:

Try clicking the wasteland to bring up the dweller exploration info screen. Wait for all your dwellers to load up, then once the arrow to browse explorers comes up (upper right corner), click your way to the waiting explorer, click collect and he/she can be clicked and assigned normally again.

If all your dwellers were either waiting or returning as the patch came out, this might not work. But fear not:

Send any dweller from your vault out to explore, then as soon as they exit recall them. Now you can choose them either directly from the line or by clicking the wasteland and the dweller exploration screen should open. Click collect and the screen will automatically jump to the next available waiting dweller. Click collect and you can assign the dweller normally again.

Still to be resolved:

If anyone has a workaround for a neglected vault, with 77 returned dwellers waiting outside the vault to be collected upon, please post here. Clicking wasteland tries to open the first waiting dweller and thus freezes the game, clicking the dweller list -sprocket in the upper left opens, but if a waiting dweller is selected the game freezes. Lastly it is not possible to send any more dwellers out because of the 25 explorer cap so can't use the tricks I used to unlock my other two vaults' bugged dwellers.

hey mate, tried both of your ways but didn't work, plus i can't sent any more dwellers to explore, because i think they reduced the number of total dwellers exploring at the same time.

i had like 30+ out and the game kept stucking when i tried to collect em. I also belive the problem to the glitch was that we had too many dwellers out at the same time (which now will make the game even more boring just because u cant have half a vault out explorin for power armors.

BUT

http://www.gamesas.com/topic/1539404-getting-frozen-after-latest-update-solution/

please visit my post and read what worked for me! (i souldn't be ctrl+c-v the whole solution here, thats why the link)

Feel free to share it, as long as it helps you as well

cya pal!