Add a null check for rootRenderer before closing the Canvas for case the renderer was never created

Author: Snipx

layout/src/main/java/com/itextpdf/layout/Canvas.java

@@ -202,7 +202,9 @@ public void flush() {
      */
     @Override
     public void close() {
-        rootRenderer.close();
+        if (rootRenderer != null) {
+            rootRenderer.close();
+        }
     }
 
     @Override