tools/script_manager.lua - added check to ensure script_data.show() runction

                           was passed as part of the script_data table
                           before trying to use it. Fixes darktable-org#418.

Author: wpferguson

tools/script_manager.lua

@@ -397,7 +397,7 @@ local function activate(script)
       if err ~= true then
         log.msg(log.debug, "got lib data")
         script.data = err
-        if script.data.destroy_method and script.data.destroy_method == "hide" then
+        if script.data.destroy_method and script.data.destroy_method == "hide" and script.data.show then
           script.data.show()
         end
       else