Using buildkit on MAMP but hitting "Fatal error: Call to a member function getDriver() on null"
This may indicate that either:


The PHP runtime is missing the PDO or MySQL extensions.  This may help: https://wiki.civicrm.org/confluence/display/CRMDOC/Setup+Command-Line+PHP (At time of writing, I'd avoid PHP 5.6 for CLI dev -- GenCode may have issues with it.)
The DB settings haven't been configured (e.g. the step for running amp config was skipped).
An incompatible or incorrect value was used in the DB settings.


For reference, I believe these are the settings for use with a typical MAMP. (Note: The line is long.)

amp config:set --db_type=mysql_dsn --mysql_dsn='mysql://root:root@127.0.0.1:3306' --httpd_type=apache --httpd_restart_command='/Applications/MAMP/Library/bin/apachectl graceful' --hosts_type=file  --perm_type=osxAcl --perm_user=www


However, if you didn't run amp config before, it's still good to run it to see more complete instructions (esp. regarding httpd.conf).