Oh try using sendMessage() instead of popup because pop ups last a very short amount of time so u could be missing it
I'm trying to fix it but I get this error "Argument 1 passed to pocketmine\command\PluginCommand::setExecutor() must implement interface pocketmine\command\CommandExecutor, instance of CustomItems\Commands given, called in C:\Users\Conner\Desktop\GensisyTestServer\plugins\TESTPLUGIN\src\CustomItems\Loader.php on line 18" (EXCEPTION) in "/src/pocketmine/command/PluginCommand" at line 73
Honestly not really this may help tho I guess idk same error it looks like https://forums.pocketmine.net/threads/unhandled-exception-error.16713/ I feel extremely stupid right now I should know this :/