Steps that caused the same type of stacktrace error:

1. Create a new workflow, drag an Assign tool on and add a new variable.
2. Right-click on Explorer and create a New Folder.
3. Click the Save button on the Menu to open the Save Dialog.
4. Notice that when you select any option on the Save Dialog, then it assumes that the user wants to move the resource, instead of just selecting it.
5. This causes the Studio to "freeze", but pressing Esc will unfreeze it.

Fix

1. Added a check on the Explorer drag and drop to check if the Save Dialog is being used to not allow drag and drop to be called.
2. We can add a CodedUI test for this by following the steps provided above. The CodedUI test should pass if the Studio does not "freeze".