onDisposeAction property Null safety
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
parameter. The platform usually shows a "Save for autofill?" prompt for
final AutofillContextAction onDisposeAction;