I have seen Avast shield crashing mainly because of conflicts. If you have any other security software running, first disable that. If possible uninstall that, Avast alone is more than sufficient to handle the threats.
If the issue is not resolved, try these steps
1) Go to Services by typing Services into the start menu (Or services.msc)
2) Find aswbIDSAgent
3) Right click on it and click Properties
4) Select the startup type as Automatic
That should resolve the issue, but if it still persists follow this guide to
fix avast shield crashes.
That should resolve your issue!