If, subsequent to the approval of an application which includes a portion of the corporate area of a city, but prior to the filing of the formation petition, such portion of the corporate area of the city is included within an existing municipal water district so that the entire corporate area of the city is included within such existing district, such portion of the corporate area of the city shall be omitted from the formation petition. Such omission shall not invalidate the approval of the application as to the remaining area of the proposed district.