Added nil safety for no user choice

Signed-off-by: Taken <taken@mairimashita.org>

lua/taken/functions/themes.lua

@@ -48,6 +48,11 @@ function M.themeselector()
     end
 
     local exec = function(choice)
+
+        if choice == nil then
+            return
+        end
+
         vim.cmd("colorscheme " .. choice)
         local newstring = 'M.colorscheme = "' .. choice .. '"'
         change_theme(oldstring, newstring)