fix Trying to access array offset on value of type null (7.4)

Author: remicollet

src/Wrapper.php

@@ -105,7 +105,7 @@ public function getProperties()
      */
     public function getProperty($name)
     {
-        return $this->schema->properties[$name];
+        return isset($this->schema->properties[$name]) ? $this->schema->properties[$name] : null;
     }
 
     /**
@@ -216,4 +216,4 @@ public function jsonSerialize()
         return $this->schema->jsonSerialize();
     }
 
-}
+}