Add isObject check to ProblemMarker

The `in` operator does not work on undefined objects.
So in order to check the node kind we need to make sure,
that the node is an object.

Contributed on behalf of STMicroelectronics

Fixes #12250

packages/markers/src/common/problem-marker.ts

@@ -16,6 +16,7 @@
 
 import { Marker } from './marker';
 import { Diagnostic } from '@theia/core/shared/vscode-languageserver-protocol';
+import { isObject } from '@theia/core/lib/common';
 
 export const PROBLEM_KIND = 'problem';
 
@@ -24,7 +25,7 @@ export interface ProblemMarker extends Marker<Diagnostic> {
 }
 
 export namespace ProblemMarker {
-    export function is(node: Marker<object>): node is ProblemMarker {
-        return 'kind' in node && node.kind === PROBLEM_KIND;
+    export function is(node: unknown): node is ProblemMarker {
+        return isObject<Marker<object>>(node) && node.kind === PROBLEM_KIND;
     }
 }