The proposition to adopt or to abandon the city manager plan of government shall not be submitted to the electors of any city later than sixty days before a regular municipal election. If, in any city, a sufficient petition is filed requiring that the question of adopting the commission plan of government, or the question of choosing a convention to frame a city charter, be submitted to the electors of such city, or if an ordinance providing for the election of a charter convention is passed by the city council, the proposition to adopt the city manager plan of government shall not be submitted in such city so long as the question of adopting the commission plan of government, or of choosing a charter convention, or adopting a charter framed by such convention, is pending.