_addContexts(oDialog) {
			oDialog.clearSelection();
			oDialog.destroyItems();
			this.oCurrentSelection = this.oSelectedContextsModel.getProperty("/selected") || [];
			return getData.call(this, {}).then(function() {
				return oDialog.open();
			});
		}
Retrieves contexts from the back end, then opens a new <code>Select Contexts</code> dialog.
@param {object} oDialog - The Select Contexts dialog
@returns {Promise} Resolves as soon as the dialog is opened