add workaround to fix NPE

Author: madmas

android-pdf-viewer/src/main/java/com/github/barteksc/pdfviewer/PDFView.java

@@ -753,6 +753,10 @@ void loadComplete(PdfFile pdfFile) {
 
         this.pdfFile = pdfFile;
 
+        if(renderingHandlerThread == null){
+            renderingHandlerThread = new HandlerThread("PDF renderer");
+        }
+
         if (!renderingHandlerThread.isAlive()) {
             renderingHandlerThread.start();
         }