Fix #1205

Author: tbar0970

vendor/phpoffice/common/src/Common/XMLWriter.php

@@ -172,11 +172,12 @@ public function writeAttributeIf($condition, $attribute, $value)
      * @param mixed $value
      * @return bool
      */
-    public function writeAttribute($name, $value)
+    public function writeAttribute($name, $value): bool
     {
         if (is_float($value)) {
             $value = json_encode($value);
         }
+        if (is_null($value)) return true;
         return parent::writeAttribute($name, $value);
     }
 }