Can you tell me which h activity is throwing that error you mentioned?
Based on the error what I see is that some variable is not getting assigned to s value. The easiest way to check this is to use the debug option.
Can you right click on the top lost activity and add a break point. Now run the the flow in debug mode. Once it reach the activity that you created the debug point it will stop highlighting the activity in yellow. While there you can also see the values assigned to the variables right below that activity is executed in the locale panel.
So, use the step into button and execute each step one by one and see whether your variables are getting assigned with some value