Display limits of email recovery

[florentos17]

tackling the issue 3219 (duplicated with the issue 3274)

[florentos17]

This curious thing is happening: when accessing the recover deleted messages panel, at first an error is thrown, but when i trigger a hot reload, then everything is fine:

IMG_0450.mov

the method is this:

String getRestorationHorizon(BuildContext context) {
    return ((_session
        ?.props[0] as Map<CapabilityIdentifier, CapabilityProperties>?)
        ?[capabilityDeletedMessagesVault]
        ?.props[0] as Map<String, dynamic>) // line causing the error
        ['restorationHorizon'];
  }

and the error is Type 'Null' is not a subtype of type 'Map<String, dynamic>' in type cast

[github-actions]

This PR has been deployed to https://linagora.github.io/tmail-flutter/3281.