The error is pretty self explanatory. When you open your workflow task, a SmartObject's load method is being called, and this load method requires a property named sharepoint url for it to work.

You should check if you have any SmartObject that is being executed when you open the task, and if you have no use for it, modify your SmartObject such that the load method does not require sharepoint url for it to execute. If you want to use the sharepoint url as a sort of filter to load some data, you will have to make sure a value is being mapped to the sharepoint url property of the SmartObject.