Handle document being null when going to page.
diff --git a/lib/src/main/java/com/artifex/mupdf/viewer/MuPDFCore.java b/lib/src/main/java/com/artifex/mupdf/viewer/MuPDFCore.java
index 387eee1..e572ecc 100644
--- a/lib/src/main/java/com/artifex/mupdf/viewer/MuPDFCore.java
+++ b/lib/src/main/java/com/artifex/mupdf/viewer/MuPDFCore.java
@@ -99,10 +99,16 @@
 			if (displayList != null)
 				displayList.destroy();
 			displayList = null;
-			page = doc.loadPage(pageNum);
-			Rect b = page.getBounds();
-			pageWidth = b.x1 - b.x0;
-			pageHeight = b.y1 - b.y0;
+			if (doc != null) {
+				page = doc.loadPage(pageNum);
+				Rect b = page.getBounds();
+				pageWidth = b.x1 - b.x0;
+				pageHeight = b.y1 - b.y0;
+			} else {
+				page = null;
+				pageWidth = 0;
+				pageHeight = 0;
+			}
 		}
 	}