I was double clicking on a empty test fixture that still appeared in the Unit Test Session. I had previously deleted the test fixture with a Safe Delete but it still hung around the session, so I clicked on the session to see why it was there. ReSharper 3.0 Version=3.0.471.2, Edition=Csharp, Built=2007-06-29T12:53:38, Configuration=ReSharper_AddIn, VsVersion=8.0 JetBrains.Util.InternalErrorException: Value cannot be null. Parameter name: solution ---> System.ArgumentNullException: Value cannot be null. Parameter name: solution at JetBrains.ReSharper.Psi.PsiManager.GetInstance(ISolution solution) in c:\Agent\work\Server\ReSharper-3.0\src\Psi\src\PsiManager.cs:line 35 at JetBrains.ReSharper.UnitTestProvider.nUnit.NUnitTestFixtureElement.GetDeclaredElement() in c:\Agent\work\Server\ReSharper-3.0\src\UnitTestProvider.nUnit\src\NUnitTestFixtureElement.cs:line 37 at JetBrains.ReSharper.UnitTestProvider.nUnit.NUnitElementBase.GetDisposition() in c:\Agent\work\Server\ReSharper-3.0\src\UnitTestProvider.nUnit\src\NUnitElementBase.cs:line 70 at JetBrains.ReSharper.UnitTestExplorer.UnitTestDescriptorBase.Navigate(TreeModelNode node, Boolean transferFocus) in c:\Agent\work\Server\ReSharper-3.0\src\UnitTestExplorer\src\Common\UnitTestDescriptorBase.cs:line 150 at JetBrains.ReSharper.TreeModelBrowser.BrowserPanel.TreeModelBrowserPanel.Navigate(TreeModelNode node, Boolean transferFocus) in c:\Agent\work\Server\ReSharper-3.0\src\TreeModelBrowser\src\BrowserPanel\TreeModelBrowserPanel.cs:line 544 at JetBrains.ReSharper.TreeModelBrowser.BrowserPanel.TreeModelBrowserPanel.NavigateNode(Object sender, TreeModelViewNavigateEventArgs e) in c:\Agent\work\Server\ReSharper-3.0\src\TreeModelBrowser\src\BrowserPanel\TreeModelBrowserPanel.cs:line 500 at JetBrains.UI.TreeView.TreeModelView.OnNavigate(TreeModelViewNavigateEventArgs e) in c:\Agent\work\Server\ReSharper-3.0\Platform\src\JetBrains.UI\src\TreeView\TreeModelView.cs:line 1417 at JetBrains.UI.TreeView.TreeModelView.HandleDoubleClick(Keys modifiers) in c:\Agent\work\Server\ReSharper-3.0\Platform\src\JetBrains.UI\src\TreeView\TreeModelView.cs:line 1429 at JetBrains.UI.TreeView.TreeModelView.OnMouseDoubleClick(MouseEventArgs args) in c:\Agent\work\Server\ReSharper-3.0\Platform\src\JetBrains.UI\src\TreeView\TreeModelView.cs:line 1316 — End of inner exception stack trace — at JetBrains.Util.Logger.LogExceptionEx(Exception, String, Boolean) in c:\Agent\work\Server\ReSharper-3.0\Platform\src\JetBrains.Util\src\Logger\Logger.cs:line 263 column 4 at JetBrains.Util.Logger.LogException(Exception) in c:\Agent\work\Server\ReSharper-3.0\Platform\src\JetBrains.Util\src\Logger\Logger.cs:line 273 column 3 at JetBrains.UI.TreeView.TreeModelView.OnMouseDoubleClick(MouseEventArgs) in c:\Agent\work\Server\ReSharper-3.0\Platform\src\JetBrains.UI\src\TreeView\TreeModelView.cs:line 1322 column 9 at System.Windows.Forms.Control.WmMouseUp(Message&, MouseButtons, Int32) at System.Windows.Forms.Control.WndProc(Message&) at DevExpress.XtraEditors.Container.EditorContainer.WndProc(Message&) at JetBrains.UI.TreeView.TreeModelView.WndProc(Message&) in c:\Agent\work\Server\ReSharper-3.0\Platform\src\JetBrains.UI\src\TreeView\TreeModelView.cs:line 1278 column 5 at System.Windows.Forms.ControlNativeWindow.OnMessage(Message&) at System.Windows.Forms.ControlNativeWindow.WndProc(Message&) at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr, Int32, IntPtr, IntPtr)