Node tree update calls in the middle of a socket loop are dangerous, they can change sockets
on group nodes and link instances in particular. Updates should only happen after the operator
has finished.
Simply removed the extra convenience check for validity now. Worst case an invalid (red) link
is created which can be removed by the user as well and should simply be ignored by node systems.
The update system in nodes needs a complete rewrite to handle complex cases like this, where an
operator may need to react to changes during its execution.
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