You cannot change objects in the Model Workspace during simulation. Or it's not intended for this at least.
Use Data Store Memory block to define a variable, and then Data Store Write at the subblock output and Data Store Read at the input of the sine block.
Instead of using a Data Store Memory block to define the data store object, you can also create a Simulink.Signal object inside your Model Workspace or Data Dictionary, it serves the same purpose. In the Data Store Write and Read blocks you will find back the Simulink.Signal object in the drop-down list.
