fix gsub to sub

init.lua

@@ -648,7 +648,7 @@ if airutils._use_signs_api then
                     if entity then
                         if entity.owner == name or minetest.check_player_privs(name, {protection_bypass=true}) then
                             if param then
-                                entity._ship_name = string.gsub(param, 1,20)
+                                entity._ship_name = string.sub(param, 1,20)
                             else
                                 entity._ship_name = ""
                             end

text.lua

@@ -176,6 +176,7 @@ local function make_text_texture(text, default_color, line_width, line_height, c
 end
 
 function airutils.convert_text_to_texture(text, default_color, horizontal_aligment)
+    text = text or ""
     default_color = default_color or 0
     horizontal_aligment = horizontal_aligment or 3
 	local font_size