The ornamental design for a casing for an electronicdevice, 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 top elevational view thereof;
FIG. 5 is a bottom elevational view thereof;
FIG. 6 is a right elevational view thereof; and,
FIG. 7 is a left elevational view thereof.
The broken line showing of structural features in included for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.