Issue #1542186 by pp, redcell, rickmanelius: Fixed Illegal string offset...

Issue #1542186 by pp, redcell, rickmanelius: Fixed Illegal string offset warning during install on sqlite.

core/lib/Drupal/Core/Database/Driver/sqlite/Update.php

@@ -34,11 +34,13 @@ class Update extends QueryUpdate {
    */
   protected function removeFieldsInCondition(&$fields, ConditionInterface $condition) {
     foreach ($condition->conditions() as $child_condition) {
-      if ($child_condition['field'] instanceof ConditionInterface) {
-        $this->removeFieldsInCondition($fields, $child_condition['field']);
-      }
-      else {
-        unset($fields[$child_condition['field']]);
+      if (isset($child_condition['field'])) {
+        if ($child_condition['field'] instanceof ConditionInterface) {
+          $this->removeFieldsInCondition($fields, $child_condition['field']);
+        }
+        else {
+          unset($fields[$child_condition['field']]);
+        }
       }
     }
   }
@@ -77,4 +79,4 @@ public function execute() {
     return parent::execute();
   }
 
-}
\ No newline at end of file
+}