enable strict null check (#65444)

src/tsconfig.strictNullChecks.json

@@ -129,6 +129,7 @@
 		"./vs/base/test/common/decorators.test.ts",
 		"./vs/base/test/common/diff/diff.test.ts",
 		"./vs/base/test/common/errors.test.ts",
+		"./vs/base/test/common/event.test.ts",
 		"./vs/base/test/common/filters.perf.test.ts",
 		"./vs/base/test/common/filters.test.ts",
 		"./vs/base/test/common/hash.test.ts",

src/vs/base/test/common/event.test.ts

@@ -71,7 +71,7 @@ suite('Event', function () {
 
 		// unhook listener
 		while (bucket.length) {
-			bucket.pop().dispose();
+			bucket.pop()!.dispose();
 		}
 
 		// noop