OR should I first uninstall avast and then try an online scanner?
It’s not necessary to uninstall any AV to try an online virus scanner 
Whilst it is not necessary to uninstal, you can pause the Standard Shield provider (right click on the avast icon > Pause Provider > Standard Shield) just for the duration of the online scan.
Some say you don’t even need to do that, but two AVs can interact and conflict, if the purpose is to confirm, then it would bebest to temporarily pause the standard shield.
Well, it’s not usual that a on-access scanner conflit with other on-line :
Maybe the exception will be Panda that does not encrypt their signature files 
I think the only advantage disabling avast will be on resources (RAM and CPU). Of course, you have a firewall and, it will be better do not browse with avast disabled… wait until the on-line scanning finishes