Exception in Benchmarks Viewer when not selecting an item in the ListBox

Description When clicking in the ListBox on the left side of Benchmarks Viewer, in the "empty part" where no project IDs are listed, the following exception is thrown:

Exception

HFM.NET v0.9.17.1040

Microsoft Windows NT 6.2.9200.0

Exception Thrown:
System.NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
   bei HFM.Forms.BenchmarksForm.listBox1_SelectedIndexChanged(Object sender, EventArgs e)
   bei System.Windows.Forms.ListBox.OnSelectedIndexChanged(EventArgs e)
   bei System.Windows.Forms.ListBox.WmReflectCommand(Message& m)
   bei System.Windows.Forms.ListBox.WndProc(Message& m)
   bei System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

Steps to reproduce

Open Benchmarks Viewer
- When no project ID is selected in the ListView, left- or right-click under the last item in the ListBox.
- When a project ID is selected, only right-clicking causes it

Classic NPE, method listBox1_SelectedIndexChanged is missing a null check for SelectedItem.

harlam357 / hfm-net

Exception in Benchmarks Viewer when not selecting an item in the ListBox #328