diff --git a/src/browser/services/CoreBrowserService.ts b/src/browser/services/CoreBrowserService.ts
index 5052de6d..f9c758c2 100644
--- a/src/browser/services/CoreBrowserService.ts
+++ b/src/browser/services/CoreBrowserService.ts
@@ -13,7 +13,7 @@ export class CoreBrowserService extends Disposable implements ICoreBrowserServic
 
   private _isFocused = false;
   private _cachedIsFocused: boolean | undefined = undefined;
-  private _screenDprMonitor = this._register(new ScreenDprMonitor(this._window));
+  private _screenDprMonitor!: ScreenDprMonitor;
 
   private readonly _onDprChange = this._register(new Emitter<number>());
   public readonly onDprChange = this._onDprChange.event;
@@ -27,6 +27,8 @@ export class CoreBrowserService extends Disposable implements ICoreBrowserServic
   ) {
     super();
 
+    this._screenDprMonitor = this._register(new ScreenDprMonitor(this._window));
+
     // Monitor device pixel ratio
     this._register(this.onWindowChange(w => this._screenDprMonitor.setWindow(w)));
     this._register(EventUtils.forward(this._screenDprMonitor.onDprChange, this._onDprChange));
diff --git a/src/tsconfig-base.json b/src/tsconfig-base.json
index e29f3b25..5ea04ea2 100644
--- a/src/tsconfig-base.json
+++ b/src/tsconfig-base.json
@@ -9,7 +9,6 @@
     "pretty": true,
 
     "incremental": true,
-    "experimentalDecorators": true,
-    "useDefineForClassFields": false
+    "experimentalDecorators": true
   }
 }