file: raise error on .on() with non-function/nil

Seems more polite than quietly accepting other types as nil.

app/modules/file.c

@@ -88,15 +88,21 @@ static int file_on(lua_State *L)
   case ON_RTC:
     luaL_unref(L, LUA_REGISTRYINDEX, rtc_cb_ref);
 
-    if ((lua_type(L, 2) == LUA_TFUNCTION) ||
-        (lua_type(L, 2) == LUA_TLIGHTFUNCTION)) {
+    switch(lua_type(L, 2)) {
+    case LUA_TFUNCTION:
+    case LUA_TLIGHTFUNCTION:
       lua_pushvalue(L, 2);  // copy argument (func) to the top of stack
       rtc_cb_ref = luaL_ref(L, LUA_REGISTRYINDEX);
       vfs_register_rtc_cb(file_rtc_cb);
-    } else {
+      break;
+    case LUA_TNIL:
       rtc_cb_ref = LUA_NOREF;
       vfs_register_rtc_cb(NULL);
+      break;
+    default:
+      luaL_error(L, "Callback should be function or nil");
     }
+
     break;
   default:
     break;

docs/modules/file.md

@@ -222,7 +222,7 @@ Trigger events are:
 
 #### Parameters
 - `event` string
-- `function()` callback function. Unregisters the callback if `function()` is omitted.
+- `function()` callback function. Unregisters the callback if `function()` is omitted or `nil`.
 
 #### Returns
 `nil`