I tried contacting customer support but they didn't seem interested since I was using the free mobile antivirus.
Anyway, I wanted to share my experience in case it helps someone.
I recently had to uninstall Avast and its products from my phone. I had been using Avast and its products for almost two years on my Sony Xperia Z3 Compact (D5803) running Android 6.0.1, and about 3 years before that on a phone running Android 4.3.1.
My wife used Google Allo to send me a message for the first time but I didn't receive it. Shortly after we talked about it, my phone displayed "Unfortunately, System UI has stopped", and then the device became unusable. The screen locked, turned black, soft buttons vanished, and the only buttons that would do anything were the power and camera buttons. I was able to reboot, but the phone would work for less than 30 seconds. I then rebooted in safe mode and experienced the same issue.
When rebooting in safe mode, there were a couple of very brief messages from Avast before the device stopped working again. I didn't get a chance to save the messages as I was in panic/fix it mode. After uninstalling Avast and its products, my phone worked again. I then received the Allo message and was able to finish configuring the app. The incident happened and was remedied 12/27 and it's been working ever since.