The ornamental design for a christmastreetopper, as shown and described.
FIG. 1 is a front perspective view of a Christmas tree topper, showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left-side elevational view thereof;
FIG. 5 is a right-side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The dashed-dot broken lines depict portions of the Christmas tree topper that form no part of the claimed design. The remaining even broken lines represent thread stitching, and are part of the claimed design.