Is this an example? I don't see where it is explained how the stargates are an example of the trope.
Even more generally, the Stargates themselves (at least the ones which use a DHD; the SGC and Atlantis ones probably have a close button.)
It's explicitly stated that a wormhole cannot remain open for more than 38 minutes (exceptions may apply in the event of black holes, Applied Phlebotinum, Negative Space Wedgies...). Other than that, a wormhole will not close while matter is in transit, which is a simple and obvious safety feature to avoid a Portal Cut. It's pretty consistent, and not too much WMG, to say that wormholes remain open until something enters them and automatically close a short time after something finishes passing through them.
Is this an example? I don't see where it is explained how the stargates are an example of the trope.