Fix a potential NPE Crash due to Mismatched Thread Lifecycle

- the thread creation and recycling was not being handled symmetrically
- this fix is based off of these two PR: #824 and mhiew#2

Author: mhiew

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

@@ -245,8 +245,6 @@ ScrollHandle getScrollHandle() {
     public PDFView(Context context, AttributeSet set) {
         super(context, set);
 
-        renderingHandlerThread = new HandlerThread("PDF renderer");
-
         if (isInEditMode()) {
             return;
         }
@@ -462,6 +460,14 @@ public void computeScroll() {
         animationManager.computeFling();
     }
 
+    @Override
+    protected void onAttachedToWindow() {
+        super.onAttachedToWindow();
+        if (renderingHandlerThread == null) {
+            renderingHandlerThread = new HandlerThread("PDF renderer");
+        }
+    }
+
     @Override
     protected void onDetachedFromWindow() {
         recycle();
@@ -753,6 +759,10 @@ void loadComplete(PdfFile pdfFile) {
 
         this.pdfFile = pdfFile;
 
+        if (renderingHandlerThread == null) {
+            return;
+        }
+
         if (!renderingHandlerThread.isAlive()) {
             renderingHandlerThread.start();
         }