optinal function in interface gives Cannot invoke an object which is possibly 'undefined' when inside a condition

I cannot removed an error even when I wrap it in a condition :

I have this in my interface :

and then in my code I do this :

I also have a condition to make sure this.customType exist

and that red line says :
Cannot invoke an object which is possibly ‘undefined’.

How is this possible if I make sure it exist in a if right before?

I’ve made a typescript code here, apparently it’s because I’m using a variable. anyway around this?
https://www.typescriptlang.org/play?ts=3.9.7#code/KYDwDg9gTgLgBASwHY2FAZgQwMbDgYQFcBnGCAW3wiXQQHNi4BvAKDjgG0BrYATwC44pKMjoBdQa3bsAbpgA2CACaZUAfgAU6eZjqDMSXgEpBAIwgR5wAwG42cAL52HLFtmqk42EmUrVaDIJEpBRUNPSMALzMLBwA5KYGBsBxEjHScHKKKqgaxEbpGXBQwDCEUEhCdhkOADRsLi5uHvDeIeQAKrxgwPgAFhbEwJXRCUlIKa4sCOhwGm2+YQHE8YlIyakAdFnKqsAFUtILof4Rq+MpYtsKu7lx8pZxRs5T0+jzPhRdPf2Dw3AAMgBXk+fnCDAO9hmc2OYOWHFh316AwgQyQVx2OX2hQysKWZ0R3WRf3R12yew090ez3sjSAA

>Solution :

In your condition, try writing it as:

if (customConfigs?.[this.customType]?.validate) {

I’d suspect the error is on the possibility of either customConfigs or customConfigs[this.customType] of being undefined

EDIT

Thanks for the playground, that highlights where the issue stands; Forget the above and simply have it as customConfigs[this.customType].validate?()

The question mark before the validate function, allow you to invoke the function only if the object beforehand exists