Add null check when determining mOnTopOfHome

    private void insertTaskAtTop(TaskRecord task) {

        // If this is being moved to the top by another activity or being launched from the home

        // activity, set mOnTopOfHome accordingly.

        final boolean fromHome = mStackSupervisor.getLastStack().isHomeStack();

        ActivityStack lastStack = mStackSupervisor.getLastStack();

        final boolean fromHome = lastStack == null ? true : lastStack.isHomeStack();

        if (!isHomeStack() && (fromHome || topTask() != task)) {

            task.mOnTopOfHome = fromHome;

        }

        mTaskHistory.remove(task);

        // Now put task at top.

        int stackNdx = mTaskHistory.size();