The ornamental design for a modular lightbar, as shown and described.
FIG. 1 is a perspective view of a modular light bar of my new design.
FIG. 2 is a front elevational view of the modular light bar of FIG. 1, the rear elevational view being a mirror image of the front elevational view.
FIG. 3 is a right side elevational view of the modular light bar of FIG. 1, the left side elevational view being a mirror image of the right side elevational view.
FIG. 4 is a top plan view of the modular light bar of FIG. 1; and,
FIG. 5 is a bottom plan view of the modular light bar of FIG. 1.