Truncate field storage tables added by module before uninstall
I was able to solve this problem by implementing hook_uninstall() in my module's install file and explicitly deleting my custom entities.

For example:

function mymodule_uninstall() {
  //remove_fields_user();

  $results = \Drupal\mymodule\Entity\MeetEntity::loadMultiple();
  foreach ($results as $entity) {
    $entity-&gt;delete();
  }
  $results = \Drupal\mymodule\Entity\Team::loadMultiple();
  foreach ($results as $entity) {
    $entity-&gt;delete();
  }
}


See Drupal 8 hook_uninstall() docs for more details