Fixed test on MaterialWindow - ensure to open() before checking the window.

Author: kevzlou7979

src/test/java/gwt/material/design/addins/client/MaterialWindowTest.java

@@ -37,6 +37,7 @@ public class MaterialWindowTest extends MaterialAddinsTest {
 
     public void init() {
         MaterialWindow window = new MaterialWindow();
+        window.open();
         checkWidget(window);
         checkStructure(window);
         checkMaximizeAndClose(window);