Hi Asad,
Can you just let me know the version of the ByD Studio which you are currently using. To check the ByD Studio 1308 Version in the following way u can execute...
1.Login to the studio
2. Go to the Help Menu in the Menu Bar
3. In Help menu go to the option of "About SAP Cloud Solutions Studio".
This will give you the version of your studio, see this if it is old version or lower version to the below shown version, kindly get a new version from the support center and install that patch into your machine. After installing this new version then try to delete the QAF screen or the entire BO or the entire project itself. This should work I hope.
Regards..
Hanu K