onAddContextsHandler() {
			this._oDialog ||= loadFragment.call(this);
			return this._oDialog.then(function(oDialog) {
				return this._addContexts(oDialog);
			}.bind(this));
		}
Proxy handler method that calls <code>_addContext</code> if the <code>Select Contexts</code> dialog is not yet opened.
@returns {Promise} Resolves as soon as the dialog is opened