Reproduction steps:
1. Open the attached project "New Unity Project (13).zip";
2. Make sure the Project View is in single-column mode;
3. Make sure the "Packages" root has its contents hidden (triangle pointing right);
4. Choose Tests>Ping Packaged Asset;

Expected results: Packages folder will be pinged.
Actual results: Only error occurs in the console and nothing happens.

Notes:
Trying the same thing with a user asset (making sure to first hide the contents of Assets, and choosing the appropriate menu item) works as expected.
This bug is not apparent in two-column mode.

Reproduced on: 2018.2.0b1, 2018.2.0b9, 2018.3.0a3. (Packages folder is showed in the Project view only from 2018.2.0b1)