I am developing a Visual Studio plugin. I want to capture the event of opening a new solution. In order to achieve this, I implemented `IVsSolutionEvents` interface and registered it using `AdviseSolutionEvents()`. However, when I run test instance of VS and open a solution, the proper event method is not being called. 

Here is my code:

```
public sealed class MyPackage : Package, IVsSolutionEvents
{
        // ...

        protected override void Initialize()
        {
            base.Initialize();

            IVsSolution solution = GetService(typeof(SVsSolution)) as IVsSolution;
            uint cookie = 0;
            solution.AdviseSolutionEvents(this, out cookie);
        }

        // ...  

        public int OnAfterOpenSolution(object pUnkReserved, int fNewSolution)
        {
            MessageBox.Show("Opened a solution!");
            return VSConstants.S_OK;
        }
}

```

Why is `OnAfterOpenSolution()` never called?