SFDX Pull fails because of AppMenu/AppSwitcher
Here is a workaround for this SFDC Platform Winter19 bug
Create/update .forceignore file at the root of your SFDX Project, with the following content :
# List files or directories below to ignore them when running force:source:push, force:source:pull, and force:source:status
# More information: https://developer.salesforce.com/docs/atlas.en-us.sfdx_dev.meta/sfdx_dev/sfdx_dev_exclude_source.htm
#
AppSwitcher.appMenu
This doesn't solve the bug, but at least the sfdx force:source:pull doesn't crash