Fix potential NPE in command error handling

Author: twisti-dev

surf-cloud-standalone/src/main/kotlin/dev/slne/surf/cloud/standalone/commands/CommandManagerImpl.kt

@@ -115,7 +115,7 @@ class CommandManagerImpl {
         } catch (e: CommandSyntaxException) {
             source.sendFailure(buildText {
                 color(Colors.ERROR)
-                appendText(e.message!!)
+                appendText(e.message ?: e.javaClass.name)
                 clickEvent(ClickEvent.suggestCommand("/$label"))
             })