fix a crash of API::root

source/gui/place.cpp

@@ -8,7 +8,7 @@
  *	http://www.boost.org/LICENSE_1_0.txt)
  *
  *	@file: nana/gui/place.cpp
- *	@contributors: qPCR4vir
+ *	@contributors: Ariel Vina-Rodriguez
  */
 
 #include <cfloat>

source/gui/programming_interface.cpp

@@ -442,7 +442,9 @@ namespace API
 	native_window_type root(window wd)
 	{
 		internal_scope_guard lock;
-		return reinterpret_cast<basic_window*>(wd)->root;
+		if(is_window(wd))
+			return reinterpret_cast<basic_window*>(wd)->root;
+		return nullptr;
 	}
 
 	window root(native_window_type wd)