FIX: Edge case attempting to destroy uninitialized tabs

Another edge case very similar to
https://github.com/silverstripe/sapphire/commit/07fb7563273fa82bd93eb255
70571c0a298198e5
Merge pull request #1739 from kinglozzer/leftandmain-uninitialized-tabs

FIX: Edge case attempting to destroy uninitialized tabs