Fix null reference when installing solution level NuGet package

Fixes #780

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

@@ -45,7 +45,12 @@ namespace ICSharpCode.PackageManagement
 		public bool AllowPrereleaseVersions { get; set; }
 		
 		public FrameworkName ProjectTargetFramework {
-			get { return Project.TargetFramework; }
+			get {
+				if (Project != null) {
+					return Project.TargetFramework;
+				}
+				return null;
+			}
 		}
 		
 		public virtual bool HasPackageScriptsToRun()

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

@@ -183,5 +183,15 @@ namespace PackageManagement.Tests
 			
 			Assert.AreEqual(expectedTargetFramework, targetFramework);
 		}
+		
+		[Test]
+		public void ProjectTargetFramework_NullProject_ReturnsNull()
+		{
+			var installAction = new InstallPackageAction(null, new FakePackageManagementEvents());
+			
+			FrameworkName targetFramework = installAction.ProjectTargetFramework;
+			
+			Assert.IsNull(targetFramework);
+		}
 	}
 }