fix: navigationview top mode

[bdlukaa]

Fixes #812
Fixes #818

Pre-launch Checklist

• I have updated CHANGELOG.md with my changes
• I have run "dart format ." on the project
• I have added/updated relevant documentation

[longfit]

Android Platform Up to Now , I cannot fix the PaneItemExpander expand the paneItem , the debugger
show:

How to avoid click PaneItemExpander face exception

void _ensureAttached() {
assert(isAttached, 'This controller must be attached to a FlyoutTarget');
}

FlyoutController.showFlyout
void _ensureAttached() {
assert(isAttached, 'This controller must be attached to a FlyoutTarget');
}