Use the same coreutils version as the rest of the app.

packages/settingeditor-extension/package.json

@@ -17,7 +17,7 @@
     "@jupyterlab/apputils": "^0.9.0",
     "@jupyterlab/codeeditor": "^0.9.0",
     "@jupyterlab/coreutils": "^0.9.0",
-    "@phosphor/coreutils": "^1.3.0",
+    "@phosphor/coreutils": "^1.2.0",
     "@phosphor/messaging": "^1.2.1",
     "@phosphor/signaling": "^1.2.1",
     "@phosphor/virtualdom": "^1.1.1",