Swipe delete, UnsupportedOperationException

[ni-ku]

Im running the example App and on the "Headers and Sections" Screen,
if i delete a single item by a swipe to the right, an UnsupportedOperationException is raised.

FATAL EXCEPTION: main
Process: eu.davidea.samples.flexibleadapter, PID: 13125
java.lang.UnsupportedOperationException
at java.util.AbstractList.set(AbstractList.java:132)
at java.util.AbstractList$ListItr.set(AbstractList.java:426)
at java.util.Collections.sort(Collections.java:247)
at eu.davidea.flexibleadapter.FlexibleAdapter.removeItems(FlexibleAdapter.java:3324)
at eu.davidea.flexibleadapter.helpers.UndoHelper.performAction(UndoHelper.java:211)
at eu.davidea.flexibleadapter.helpers.UndoHelper.start(UndoHelper.java:198)
at eu.davidea.samples.flexibleadapter.MainActivity.onItemSwipe(MainActivity.java:791)
at eu.davidea.flexibleadapter.FlexibleAdapter.onItemSwiped(FlexibleAdapter.java:4672)
at eu.davidea.flexibleadapter.helpers.ItemTouchHelperCallback.onSwiped(ItemTouchHelperCallback.java:249)

Checking the size of the items positions list seems to fix the issue.
`

    if (selectedPositions.size() >= 2) {
        // Reverse-sort the list, start from last position for efficiency
        Collections.sort(selectedPositions, new Comparator<Integer>() {
            @Override
            public int compare(Integer lhs, Integer rhs) {
                return rhs - lhs;
            }
        });
        log.v("removeItems after reverse sort selectedPositions=%s", selectedPositions);
    }`

[davideas]

Thanks @ni-ku, it is caused because of the singletonList, created in MainActivity, is an immutable list. However, I will put also that check. The issue remains if the list is immutable with more than 1 item, but then the exception is correct.
This will be a small improvement and not a bug.