When I empty the main compartment of my V15 detect plus the entire cartridge comes apart from the machine.

On the app there is a video on how to replace the main body, but I cannot emptying the vacuum without it disassembling itself which is very messy. 
 

How can this be fixed? It seems like whatever part is supposed to stop it coming off unless the red button with the 1 on it is pressed isn’t working.