The battle of Null was fought during the Clone Wars after the planet Null joined the Confederacy of Independent Systems.
In 22 BBY, Null had become a battlezone between Republic and Separatist forces. A triage unit overseen by Jedi healers was set up behind Republic lines, though by this point, it was in danger of being over-run by enemy forces.
Barriss Offee defending her patients.
While attempting to heal wounded clone troopers and Jedi, T'ra Saa and Barriss Offee were caught in an unexpected attack by bounty hunters working for the Crimson Nova chapter of the Bounty Hunters' Guild. The hunters, riding speeder bikes, opened fire on the Jedi, killing some and setting fire to the surrounding forest. During the fight, T'ra Saa's lightsaber was knocked from her hand and collected by Nazzer, a hunter previously encountered by Offee and her master Luminara Unduli, who ultimately fled the encroaching fires. Left with no alternative against the blaze, Saa used the Force to pull trees down over the survivors, effectively trapping them away from the flames.
A rainstorm soon doused the fires, and Mace Windu arrived on the scene to rescue the survivors. He uncovered Offee, the clones and an unconscious Saa, who had been wounded during her valiant attempt to save the group. Windu raced her to a waiting gunship along with Offee, retreating as Separatists were spotted approaching their location. The Jedi Healer informed Windu of the details of the unexpected battle, and the Jedi Master resolved to put an end to the Crimson Nova's activities with a mission to the Rig, their base of operations.
While the 2004 Dark Horse Comics issue Republic 65 originally established the battle of Null to have occurred seventeen months after the Battle of Geonosis, changes to the timeline of the Clone Wars necessitated by the Star Wars: The Clone Wars multimedia project moved the battle back to 22 BBY.