Hello ErkangHui,
The assignee can be unassigned by removing the user from the assignee association.
Opening a workflow by the first user (Mendix version 9.24.15, in newer versions there might be more other, more suitable options). Workflows and usertasks are created by usage of the queue mechanism. Which means that if you start a workflow and you want to open it, it might not be created yet due the queue.
Work around for this (see screenshot):
Call workflow = subflow where you call your workflow
EndTransaction = java action can be found at the communitycommons module
Wait microflow = microflow where you loop x times to wait till the object is created
Wait microflow:
If the workflowusertask is returned you can open the page, if not you can send the user a notification that he or she should open it from the taskinbox.
In our case we loop 12 times 250ms so in the worst case scenario an user waits 3 seconds. However during our testing period we never had any issues and the user never had any issues with a noticeable delay (the task was always found after the first try). We tested with robotframework and simulated our usergroup.
Note: for our usecase it was a requirement to starting within the workflow and a solution that worked. That does not mean that this is the best solution for you, for example another option can be to have a start with a data entry page which from there can trigger the workflow (step 2, which is actually the first step within the workflow itself).
Good luck.
Thijs