_bootstrap_layouts_preprocess_layout fails to find layout if custom template is used

I'm using a number of custom layouts that use templates from this module, which work, like so:

mytheme_basic:
  label: Basic
  description: 'Standard layout for most content.'
  category: MyTheme
  class: '\Drupal\bootstrap_layouts\Plugin\Layout\BootstrapLayoutsBase'
  type: partial
  path: ../../modules/contrib/bootstrap_layouts/
  template: templates/3.0.0/bs-2col-stacked
  library: mytheme/layouts
  regions:
    top:
      label: 'Top'
      classes:
        - col-sm-12
    left:
      label: 'Left'
      classes:
        - col-sm
    right:
      label: 'Right'
      classes:
        - col-sm-4
    bottom:
      label: 'Bottom'
      classes:
        - col-sm-12

But when I create a custom template for one of these layouts, I get "Fatal error: Call to a member function id() on null in /code/modules/contrib/bootstrap_layouts/bootstrap_layouts.module on line 83". It's trying to reference the layout at $variable['layout'] but it's no longer there for some reason. This is the layout causing that error:

mytheme_list_item:
  label: List Item
  description: 'Layout for display in a list.'
  category: MyTheme
  class: '\Drupal\bootstrap_layouts\Plugin\Layout\BootstrapLayoutsBase'
  type: partial
  template: templates/layout/sprout-list-item
  library: ma_bootstrap/sprouts-layouts
  regions:
    top:
      label: 'Top'
      classes:
        - col-sm-12
    left:
      label: 'Left'
      classes:
        - col-sm
    right:
      label: 'Right'
      classes:
        - col-sm
    bottom:
      label: 'Bottom'
      classes:
        - col-sm-12