The ornamental design for a pop up shelter, as shown and described.
FIG. 1 is a perspective view showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line showing of portions of the pop up shelter is included for the purpose of illustrating environmental structure and forms no part of the claimed design.
The fragmentary cross-hatch shading represents fabric and is understood to extend uniformly throughout the area represented.