Enable strict null checking for GridView test

src/tsconfig.strictNullChecks.json

@@ -104,6 +104,7 @@
 		"./vs/base/test/browser/progressBar.test.ts",
 		"./vs/base/test/browser/ui/contextview/contextview.test.ts",
 		"./vs/base/test/browser/ui/grid/grid.test.ts",
+		"./vs/base/test/browser/ui/grid/gridview.test.ts",
 		"./vs/base/test/browser/ui/grid/util.ts",
 		"./vs/base/test/browser/ui/list/listView.test.ts",
 		"./vs/base/test/browser/ui/list/rangeMap.test.ts",

src/vs/base/test/browser/ui/grid/gridview.test.ts

@@ -22,7 +22,7 @@ suite('Gridview', function () {
 	});
 
 	teardown(function () {
-		gridview = null;
+		gridview = null!;
 	});
 
 	test('empty gridview is empty', function () {