Expression Changed AfterIt Has Been Checked when calling last function in *ngFor

Continuing the discussion from Instant scroll to bottom after Firebase load:

I’m trying to scroll to the bottom with extreme problems.
I have tried the solution and in Android seems to work but I have a problem in iOS: the function for last item is not fired (why ?).
But the strage things is that this function is called 20 times for my 4 items. How can is it possible ? Why ? If I try to limit it using a flag I got:

Error: ExpressionChangedAfterItHasBeenCheckedError: Expression has changed after it was checked. Previous value: 'true'. Current value: 'false'.

could you show use the html