onDisposeAction property Null safety

The AutofillContextAction to be run when this AutofillGroup is the topmost AutofillGroup and it's being disposed, in order to clean up the current autofill context.

An autofill context is a collection of input fields that live in the platform's text input plugin. The platform is encouraged to save the user input stored in the current autofill context before the context is destroyed, when TextInput.finishAutofillContext is called with shouldSave set to true.

Currently, there can only be at most one autofill context at any given time. When any input field in an AutofillGroup requests for autofill (which is done automatically when an autofillable EditableText gains focus), the current autofill context will merge the content of that AutofillGroup into itself. When there isn't an existing autofill context, one will be created to hold the newly added input fields from the group.

Once added to an autofill context, an input field will stay in the context until the context is destroyed. To prevent leaks, call TextInput.finishAutofillContext to signal the text input plugin that the user has finalized their input in the current autofill context. The platform text input plugin either encourages or discourages the platform from saving the user input based on the value of the shouldSave parameter. The platform usually shows a "Save for autofill?" prompt for user confirmation.

Defaults to AutofillContextAction.commit, which prompts the platform to save the user input and destroy the current autofill context.