Error "Unable to load schema from 'apprt://.manifest.schema.json'" displayed when editing manifest.json file

[ctjdr]

Describe the bug
Sometimes, when editing a manifest.json file, VS Code displays the following error in the editor:

Error "Unable to load schema from 'apprt://.manifest.schema.json'"

This is causes by the SchemaDocumentContentProvider being registered too late, when VS Code is started with a manifest.json files already opened.

In microsoft/vscode#127812 there is a solution described by adding "onFileSystem:yourScheme" to the list of activation events for an extension.