Fix #563 null reference exception when managing packages.

A null reference exception would occur if the Manage Packages dialog
was opened when the solution was selected, the solution contained
only one project, and then a package was installed. If the solution
had more than one project the exception would not occur.

The NuGet addin was incorrectly treating a solution with one project
the same as if the project was selected when the Manage Packages
dialog was opened.

src/AddIns/Misc/PackageManagement/Project/Src/PackageViewModel.cs

@@ -219,7 +219,7 @@ namespace ICSharpCode.PackageManagement
 
 		public void AddOrManagePackage()
 		{
-			if (selectedProjects.HasMultipleProjects()) {
+			if (!selectedProjects.HasSingleProjectSelected()) {
 				if (IsProjectPackage(package)) {
 					ManagePackage();
 				} else {

src/AddIns/Misc/PackageManagement/Test/Src/PackageViewModelTests.cs

@@ -1531,5 +1531,30 @@ namespace PackageManagement.Tests
 			
 			operationAwareRepository.AssertOperationWasStartedAndDisposed(RepositoryOperationNames.Install, "MyPackage");
 		}
+		
+		[Test]
+		public void AddOrManagePackage_SolutionSelectedAndContainsOnlyOneProject_UserPromptedToSelectOneProject()
+		{
+			CreateFakeSolution();
+			AddProjectToSolution();
+			fakeSolution.FakeMSBuildProjects[0].Name = "MyProject";
+			fakeSolution.NoProjectsSelected();
+			fakeSolution.AddFakeProjectToReturnFromGetProject("MyProject");
+			CreateViewModel(fakeSolution);
+			UserCancelsProjectSelection();
+			
+			viewModel.AddOrManagePackage();
+			
+			IEnumerable<IPackageManagementSelectedProject> selectedProjects = 
+				fakePackageManagementEvents.SelectedProjectsPassedToOnSelectProjects;
+			
+			var expectedSelectedProjects = new List<IPackageManagementSelectedProject>();
+			expectedSelectedProjects.Add(new FakeSelectedProject("MyProject"));
+			
+			var nullReferenceException = new NullReferenceException();
+			Assert.IsFalse(fakeLogger.FormattedMessagesLoggedContainsText(nullReferenceException.Message));
+			Assert.IsNotNull(selectedProjects);
+			SelectedProjectCollectionAssert.AreEqual(expectedSelectedProjects, selectedProjects);
+		}
 	}
 }