4909150: WindowsTreeUI can cause NullPointerException occasionally

Reviewed-by: alexp

src/share/classes/com/sun/java/swing/plaf/windows/WindowsTreeUI.java

@@ -81,6 +81,7 @@ public class WindowsTreeUI extends BasicTreeUI {
             else {
                 Rectangle   beginRect = getPathBounds(tree, getPathForRow
                                                       (tree, beginRow));
+                if (beginRect != null) {
                     Rectangle   testRect = beginRect;
                     int         beginY = beginRect.y;
                     int         maxY = beginY + visRect.height;
@@ -88,15 +89,17 @@ public class WindowsTreeUI extends BasicTreeUI {
                     for(int counter = beginRow + 1; counter <= endRow; counter++) {
                         testRect = getPathBounds(tree,
                                                  getPathForRow(tree, counter));
-                    if((testRect.y + testRect.height) > maxY)
+                        if(testRect != null && (testRect.y + testRect.height) > maxY) {
                             counter = endRow;
                         }
+                    }
                     tree.scrollRectToVisible(new Rectangle(visRect.x, beginY, 1,
                                                       testRect.y + testRect.height-
                                                       beginY));
                 }
             }
         }
+    }
 
     static protected final int HALF_SIZE = 4;
     static protected final int SIZE = 9;