b-toast notification is overlay in modal in bootstrap-vue in VUEJS

Question

This is picture

I'm using $bvToast.toast() for using toast notification. This is some line of code

 this.$bvToast.toast('Hit enter to submit\n Click outside to cancel', {
        title: 'Variant info',
        autoHideDelay: 2000,
        appendToast: true,
        solid: true,
        toastClass: {zIndex:9999999}
      })

I think it's kind of having problem with z-index but I do not how to override z-index for b-toast? I saw some property toastClass, I set toastClass:{zIndex:9999}, but it still not work.

Edit 1:I just add toastIndex in data and use it like in -pic 2-. Is that right? I'm not familiar with syntax? full pic pic 1

pic 2