fix: an issue that crashes when cancel is not present (#852)

index.js

@@ -129,7 +129,7 @@ export default class Pdf extends Component {
 
         if ((nextSource.uri !== curSource.uri)) {
             // if has download task, then cancel it.
-            if (this.lastRNBFTask) {
+            if (this.lastRNBFTask && this.lastRNBFTask.cancel) {
                 this.lastRNBFTask.cancel(err => {
                     this._loadFromSource(this.props.source);
                 });