I found two little opportunities to be null safe and theoretically avoid creating a local closure.

In the first, we create a local variable, final mainIsolate, and then create a large local closure, callExtension. Then we check if that local variable is null, in order to return early. Moving that check, and one more, above the closure declaration adds three benefits: (1) the variable is promoted to be non-null, (2) the code reads a little easier, grouping these return early checks, and (3) we avoid creating the local closure when we don't need it, potentially saving memory.

The second is very similar to the first.