Dropdown Knobs crash the storybook if an invalid value is typed.

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Description

Dropdown Knobs crash the storybook if an invalid value is typed.

Steps To Reproduce

1. Add a list knob to your story, (The TextScale is used as an example)

2. Type into the box any invalid value.

3. Press enter

4. Bang

════════ Exception caught by widgets ═══════════════════════════════════════════
The following _TypeError was thrown while calling onEditingComplete for TextInputAction.done:
Null check operator used on a null value

When the exception was thrown, this was the stack:
#0 ListField.toWidget. (package:widgetbook/src/fields/list_field.dart:41:63)
list_field.dart:41
#1 _DropdownMenuState.build.. (package:flutter/src/material/dropdown_menu.dart:695:36)
dropdown_menu.dart:695
#2 EditableTextState._finalizeEditing (package:flutter/src/widgets/editable_text.dart:3277:34)
editable_text.dart:3277
#3 EditableTextState.performAction (package:flutter/src/widgets/editable_text.dart:3140:9)
editable_text.dart:31

Expected Behavior

The app does not crash, and instead rejects the input in some way.