Merge "Adjust type hints in htmlform related classes"

diff --cc includes/htmlform/HTMLForm.php
index 99e387a,86f9dba..a7cef3c
--- 1/includes/htmlform/HTMLForm.php
--- 2/includes/htmlform/HTMLForm.php
+++ b/includes/htmlform/HTMLForm.php
@@@ -180,10 -180,12 +180,11 @@@ class HTMLForm extends ContextSource 
        protected $mMessagePrefix;
  
        /** @var HTMLFormField[] */
 -      protected $mFlatFields;
 -
 -      protected $mFieldTree;
 +      protected $mFlatFields = [];
 +      protected $mFieldTree = [];
        protected $mShowReset = false;
        protected $mShowSubmit = true;
+       /** @var string[] */
        protected $mSubmitFlags = [ 'primary', 'progressive' ];
        protected $mShowCancel = false;
        protected $mCancelTarget;