Supergirl season 5 hasn't done any obvious build-up for Crisis on Infinite Earths, but it may secretly be setting up its connection to the Arrowverse crossover through one of the main heroes. Since the DC TV shows returned, the new seasons of Arrow and The Flash have been doing the legwork for the highly anticipated event, but as Supergirl is involved in - and even starting - the event, it makes sense for it to include some of its own setup.
In the 12-issue comic that the crossover is based on, Kara Zor-El played a massive role in the Crisis story that changed DC Comics forever. The run saw the tragic but heroic death of Superman's famous cousin, which the Arrowverse has hinted at to a degree. Supergirl will be the first series to kick-off the Crisis starting on Sunday, December 8. Despite the Monitor (LaMonica Garrett) unleashing Malefic (Phil LaMarr) in the season 4 finale, the godlike being hasn't been seen at all this season. With all of these factors not being taken in, the show may have another way to set up its involvement with the Multiverse story.
Introduced in the fourth season, Nia Nal a.k.a. Dreamer (Nicole Maines) has quickly grown to become one of Kara's Super-Friends. Having the ability to dream about the future, Nia has come to be a powerful player in the DC series. With the Multiverse at stake, it's only a matter of time before the Supergirl characters become aware of the deadly danger. Since Arrow and The Flash are respectively handling the Crisis in their own way, Supergirl could potentially be following suit. The closer the Anti-Monitor gets to Earth-38, the higher the chances are that Nia will be the first character to learn about the Crisis. With only three episodes remaining before Crisis on Infinite Earths starts on Supergirl, Nia will possibly witness the threat in her dreams.
Given how Supergirl has established Nia's precognitive abilities, dreaming the Multiverse being in danger would only be natural for the heroine. Whether it's their Earth being destroyed or merged with Earth-1 remains to be seen. But having Nia be the one to alarm the team allows the series to set up their involvement in the crossover in a unique way. On Arrow and The Flash, it was through the Monitor that the heroes learned about what will soon happen to the Multiverse.
It's possible that the Monitor will show up in National City again before the crossover begins. But having Nia seeing the Crisis in her dreams would give her a big role prior to the event. and tie Supergirl in more directly. As the current Arrowverse seasons progress, the closer Crisis on Infinite Earths gets to bringing all of the shows together for the biggest challenge they have ever faced.