Fix mTempFiles being null when inside a tab view on Android

[mystborn]

When a DocumentView is inside of a BottomTabNavigator from @react-navigation/bottom-tabs, it will be detached from the window without being completely destroyed. The next time the user goes to the tab, the app will crash because mTempFiles was set to null inside of onDetachedFromWindow

pdftron-react-native/android/src/main/java/com/pdftron/reactnative/views/DocumentView.java

Line 2356 in 642b707

mTempFiles = null;

but onAttachedToWindow still tries to read it:

pdftron-react-native/android/src/main/java/com/pdftron/reactnative/views/DocumentView.java

Line 2285 in 642b707

mTempFiles.add(new File(fileUri.getPath()));

To fix, I just added a safety check to make sure the field is initialized before use inside of onAttachedToWindow.

[sgong-pdftron]

Closing in favor of #702