Is it safe to stop the shrinking of MSDB?

In a production server (Microsoft SQL Server 2014), the file MSDBData.mdf ballooned up to more than 450 GB, eating almost all the space in the server (only 38 GB remaining). First thing, I created a backup of MSDB on a network share (via _SSMS -> activities -> backup_). Then I tried to reduce the size of the file (via SSMS -> activities-> Shrink -> file), but the shrinking is stalled and no disk space has been released. The shrinking dialog window is frozen since hours. In the activities monitoring of SSMS, in processes monitoring the shrinking appears to be in SUSPENDED status. In Event viewer there are several events i.e.

(source MSSQLSERVER, eventid 847) beginning with

 1. Time-out occurred while waiting for latch: class 'FGCB_ADD_REMOVE'
 2. Time-out occurred while waiting for latch: class 'FCB'

The question: is it safe to close the dialog window of SSMS to stop the shrinking of MSDB, in order to carry out further investigation?