3165 fix out transition

[matyunya]

#3165

Fix for broken out page transition when outro.callbacks is undefined.

[neurosoup]

if (outroing.has(block) || !outros || !outros.callbacks) return;

should be more secure. ;)

[matyunya]

Is there a chance for it to get merged either way? I spent a few hours trying to do a proper fix (meaning outros.callbacks is pushed to while outros is undefined, so group_outros has not been called. But looks like it's some really obscure scenario :(

[matyunya]

oh, and outros is declared in the upper scope.

[matyunya]

ceff403#diff-2cf1e273e0de5eb82ca2e4facf6c913eL27