StartUnityAction from OpenUnityProjectAsFolderNotification wouldn't fully work in Unity 2019.2+

com.jetbrains.rider.plugins.unity.actions.StartUnityAction.Companion#startUnityAndRider
would result
```
executeMethod class 'RiderMenu' could not be found.
Argument was -executeMethod JetBrains.Rider.Unity.Editor.RiderMenu.MenuOpenProject
```
in Unity.

because EditorPlugin is not loaded at the moment, when this code tries to execute.

to reproduce:
1. Remove sln from Unity project
2. Open as folder in Rider
Notification is shown.
3. Click link in the notification.
Get error in Unity.

Start at com.jetbrains.rider.plugins.unity.actions.StartUnityAction.Companion#startUnityAndRider and reproduce by removing the solution, opening the project as a folder in Rider, and clicking the notification link in Unity 2019.2+. Check when the Unity EditorPlugin is loaded; done means the action opens the project without the RiderMenu class-not-found error.

Written by the indexing model from the issue text.