The problem seems to be in how the sendHttpPostRequest action works in OH2. You can try putting quotations around json string. This will get rid of the exception. However, for some reason I am not seeing the post reaching Dashing using sendHttpPostRequest. For now, I’ve reverted back to executeCommand and using curl.
This is not ideal (it requires curl to be installed) and I will look further into it when I get time, and will post updates to my main thread (https://community.openhab.org/t/dashboard-ui-suitable-for-tablets/2329)