We're just getting started at detecting gravitational waves. So eventually we'll be able to detect a wandering gravitational body heading towards us. Even if one is heading towards us, it'll take millions or billions of years to get to us.

Black holes do not exist, so a wandering mythical black hole will never swallow the Earth.

This is essentially just silly scientific theory. They have been trying for so long to prove this nonsense, that they are determined at all costs to label any compelling circular, helical or globular interstellar formation, a black hole.
Time for astrophysicists to restart and go back to the drawing board on that one.

It's possible. But, the nearest one is many light years away - and, moving in about the same direction and similar speed as we do around the galactic center - so, for the foreseeable future, we're not in danger of being visited by a black hole.