Try to close your modal inside a finalize() :

onSubmit() {
isError = false;
...
            this.updateSub = this.bewirtungService.updateCatering(cateringCreate)
.pipe(
finalize(()=> {
    if(!isError) {
    this.dialogRef.close(b);
    }
})
)
.subscribe(
                (b: Bewirtung) => {
                    ...
                    // don't close it here
                },
                () => {
                    isError = true;
                    this.notificationService.error('Die Bewirtung konnte nicht geändert werden.');
                }
            );
        }
        else {...}
    }