Catch edge case where OS mappings weren't set

src/SearchAPI/BackendClient.php

@@ -372,6 +372,11 @@ class BackendClient implements BackendClientInterface {
       'index' => $this->getIndexId($index),
     ]);
     $drupalMapping = $this->fieldParamsBuilder->mapFieldParams($this->getIndexId($index), $index);
+
+    // If mappings have yet to be set no need to clear.
+    if (!isset($openSearchMapping[$this->getIndexId($index)]['mappings']['properties'])) {
+      return FALSE;
+    }
     // Recursively check for differences in the mappings between the
     // $openSearchMapping and $drupalMapping arrays. Before comparing,
     // convert $drupalMapping to a json format returned as an