Phone Reboots on Installing/Launching Certain Apps
This could solve the problem temporarily. I had the same problem with my own phone and it appears to be a material problems, many recent updates are incompatible with some phones (it is the case for wiko phones).
Bypassig the play store and installing a previous update can work. For whatsapp you will need a previous version than the actual one (2.18.64), but that will be older than the 2.18.0, otherwise whatsapp won't let you start the app.

For example downloading this version and manually install worked for me: https://apkpure.com/whatsapp-business/com.whatsapp.w4b/download/164-APK

EDIT:

Now this version is considered obsolete by whatsapp too. The only way to continue using it for me was to set the date of my phone to a previous date that the date showed on whatsapp saying "This version of whatsapp is obsolete since date".

I hope this problem will be fixed later because it is unsafe to install previous versions, generally updates include bug and security fixes.