Fix null reference exception when installing/uninstalling NuGet packages a project is not selected and the solution contains only one project.

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

@@ -288,7 +288,12 @@ namespace ICSharpCode.PackageManagement
 		}
 		
 		public bool IsManaged {
-			get { return selectedProjects.HasMultipleProjects(); }
+			get {
+				if (selectedProjects.HasMultipleProjects()) {
+					return true;
+				}
+				return !selectedProjects.HasSingleProjectSelected();
+			}
 		}
 		
 		public void ManagePackage()

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

@@ -766,6 +766,19 @@ namespace PackageManagement.Tests
 			Assert.IsTrue(managed);
 		}
 		
+		[Test]
+		public void IsManaged_SolutionSelectedContainingOneProject_ReturnsTrue()
+		{
+			CreateFakeSolution();
+			AddProjectToSolution();
+			fakeSolution.NoProjectsSelected();
+			CreateViewModel(fakeSolution);
+			
+			bool managed = viewModel.IsManaged;
+			
+			Assert.IsTrue(managed);
+		}
+		
 		[Test]
 		public void IsManaged_SolutionWithOneProjectSelected_ReturnsFalse()
 		{