Fix crash calling openDirectories() in various cases

Fix crash calling openDirectories() in various cases

This can cause a crash when called with a single url and split view, or
when the last URL is not shown, because the iterator gets mutated in
the loop, so we need to make sure it hasn't gone out of bounds. This
check was erroneously removed in 175538020824355115cd98637ce83ecc53badd44
and needs to be brought back.