This San Francisco Chronicle article quotes several leaders of the California Republican Party on the problem that the top-two system will likely result in no Republican appearing on the November 2018 ballot.
Jim Brulte, chair of the state party, is quoted as saying the November election is a “run-off.” He is incorrect. A run-off is something held after an election, in cases when the election failed to produce a winner. But in California, the June primary is not an election, because no one can be elected in June (for congress and partisan state office). Federal law forbids states from holding elections for congress at any time other than November of even-numbered years (except for special elections). States that want run-offs must hold them after November. The only two states with congressional general election run-offs are Georgia and Louisiana. Georgia holds them in January of odd years, and Louisiana in December of even years.