The ornamental design for a portablelantern, as shown and described.
FIG. 1 is a top, front, left perspective view of a portable lantern showing my design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left elevational view thereof;
FIG. 5 is a right elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines immediately adjacent the shaded surfaces depict the bounds of the claim that forms no part of the claimed design. All other broken lines depict portions of the portable lantern that form no part thereof.