All you have to do is validate your value within the function. Old school.
the difference is you cannot set the pointer for the animation sequence to a pre-set, it needs a specific input or selection to work.
As such you cannot make the node “just work” without giving the user an error when he doesn’t connect it to the correct data - or select the animation sequence.
within the function, check if any of the variables are null, if they are, return a custom error.
since you are also outputting a float in your case, you may want to return that value as 0 or -1 depending on what fits the usage.
UE_LOG(Fatal, would cause the blueprint compiler to stop and report an error I believe.
There are some pre built macros you can also use in
Engine\Source\Runtime\Core\Public\Misc\AssertionMacros.h
other then that, it’s really up to the end user to make sure they use your functions correctly…