fix ios entry (#3027)

When initWithFrame is being called s_ctx has not been created yet. Fix with correct assign order.

examples/common/entry/entry_ios.mm

@@ -212,8 +212,6 @@ using namespace entry;
 		return nil;
 	}
 
-	s_ctx->m_window = self.layer;
-
 	return self;
 }
 
@@ -335,6 +333,7 @@ using namespace entry;
 	[m_view setContentScaleFactor: scaleFactor ];
 
 	s_ctx = new Context((uint32_t)(scaleFactor*rect.size.width), (uint32_t)(scaleFactor*rect.size.height));
+	s_ctx->m_window = m_view.layer;
 	return YES;
 }