QuickFix to generate OnValidate IsNotNull on SerializeField

```
public class NewBehaviourScript : MonoBehaviour
{
[SerializeField] private GameObject MyField;

// QuickFix to generate this on MyField
void OnValidate()
{
Assert.IsNotNull(MyField);
}
}
```

Start by locating the existing QuickFix entry points for Unity and serialized fields, then review how generated methods and assertions are tested. Done means a QuickFix can generate the shown OnValidate method with an Assert.IsNotNull check for a selected SerializeField.

Written by the indexing model from the issue text.