Fix null reference exception in Ruby test teardown.

src/AddIns/BackendBindings/Scripting/Test/Designer/LoadFormTestsBase.cs

@@ -28,7 +28,9 @@ namespace ICSharpCode.Scripting.Tests.Designer
 		[TestFixtureTearDown]
 		public void TearDownFixture()
 		{
-			form.Dispose();
+			if (form != null) {
+				form.Dispose();
+			}
 		}
 
 		/// <summary>