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NullPointerException initStickyHeadersHolder on orientation change

See original GitHub issue

Situation:

Launch Activity starts Main activity
Fragment added in activity on created
Adapter initialized onViewCreated
Set stickyHeaders(true)
Rotate the device while Main activity has not fully created (timing is difficult, use debugger to stop onCreate and then rotate device)
Fragment gets recreated together with the adapter
Old adapter execute posted runnable and tries to add sticky header in a null view

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.ViewGroup.addView(android.view.View)' on a null object reference
                                                                        at eu.davidea.flexibleadapter.helpers.StickyHeaderHelper.initStickyHeadersHolder(StickyHeaderHelper.java:104)
                                                                        at eu.davidea.flexibleadapter.helpers.StickyHeaderHelper.attachToRecyclerView(StickyHeaderHelper.java:77)
                                                                        at eu.davidea.flexibleadapter.FlexibleAdapter$5.run(FlexibleAdapter.java:1357)
                                                                        at android.os.Handler.handleCallback(Handler.java:739)
                                                                        at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                        at android.os.Looper.loop(Looper.java:158)
                                                                        at android.app.ActivityThread.main(ActivityThread.java:7225)
                                                                        at java.lang.reflect.Method.invoke(Native Method)
                                                                        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                                                                        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)

The problem is this piece of code

    public FlexibleAdapter<T> setStickyHeaders(final boolean sticky, @Nullable ViewGroup stickyContainer) {
        log.i("Set stickyHeaders=%s (in Post!)%s", sticky, (stickyContainer != null ? " with user defined Sticky Container" : ""));

        // With user defined container
        mStickyContainer = stickyContainer;

        // Run in post to be sure about the RecyclerView initialization
        mHandler.post(new Runnable() {
            @Override
            public void run() {
                // Enable or Disable the sticky headers layout
                if (sticky) {
                    if (mStickyHeaderHelper == null) {
                        mStickyHeaderHelper = new StickyHeaderHelper(FlexibleAdapter.this,
                                mStickyHeaderChangeListener, mStickyContainer);
                        mStickyHeaderHelper.attachToRecyclerView(mRecyclerView);
                        log.i("Sticky headers enabled");
                    }
                } else if (areHeadersSticky()) {
                    mStickyHeaderHelper.detachFromRecyclerView();
                    mStickyHeaderHelper = null;
                    log.i("Sticky headers disabled");
                }
            }
        });
        return this;
    }

Since you are posting a runnable if the timing is good it will be executed after onDestroyView.

Possible solutions, check if recylcer is still there or remove the runnable.