Make sure the provided models actually exist.

includes/ingest.form.inc

@@ -78,6 +78,12 @@ function islandora_ingest_form_init_form_state_storage(array &$form_state, array
     if (empty($objects)) {
       $objects[] = islandora_ingest_form_prepare_new_object($configuration);
     }
+    // Make sure the models actually exist
+    foreach ($configuration['models'] as $key => $model) {
+      if (!islandora_object_load($model)) {
+        unset($configuration['models'][$key]);
+      }
+    }
     // No need to persist the 'objects' within the configuration.
     unset($configuration['objects']);
     // Required for step hooks.