Trivially reproduced as follows (code also attached):

import QtQuick 2.0
import QtLocation 5.4

Rectangle {
    width: 800
    height: 600

    Component.onCompleted: {
        map.clearMapItems();
    }

    /* NOTE: No plugin name is set. */
    Plugin { id: plugin }

    Map {
        id: map

        anchors.fill: parent
        plugin: plugin
    }
}

This is due to QDeclarativeGeoMap::clearMapItems calling m_map->clearMapItems() without checking if m_map is non-NULL. Looks to be a non-issue in Qt 5.10 and I believe the problem was introduced in Qt 5.9. Not sure if further 5.9 point releases are expected.