wagtail - How to get an page instance from a block instance on a admin edit page?

Question

I have a custom wagtail PageChooserBlock.

class SelectThroughBlock(wblocks.PageChooserBlock):
    @cached_property
    def field(self):
        from django import forms
        return forms.ModelChoiceField(
            queryset=self.target_model.objects.all(), widget=self.widget, required=self.required,
            help_text=self.help_text)

    @cached_property
    def widget(self):
        from django.forms.widgets import Select
        return Select()

How can I get an instance of edit page in wagtail admin?