QTreeView with QAbstractItemModel - insert rows causes crash

from PyQt4.QtCore import *
from PyQt4.QtGui import *
from lsfw2.lsgui.tree import TreeModel, TreeItem

class MyTreeItem(TreeItem):

    def __init__(self, data, parent=None):
        TreeItem.__init__(self, data, parent)
        if parent is not None:
            parent.appendChild(self)


class MyProxyModel(QSortFilterProxyModel):

    def __init__(self, *args, **kwargs):
        super(MyProxyModel, self).__init__(*args, **kwargs)
        self._filter_wildcard = ''

    def filterAcceptsRow(self, source_row, source_parent):
        index = self.sourceModel().index(source_row, 0, source_parent)
        item = self.sourceModel().get_item(index)
        if item is None:
            return False
        return self.is_visible(item)

    def is_visible(self, item):
        items = [item]
        items.extend(item.childItems)
        for i in items:
            if self._filter_wildcard in unicode(i.data(0)).lower():
                return True
        return False

    def setFilterWildcard(self, filter_wildcard):
        self._filter_wildcard = filter_wildcard
        return super(MyProxyModel, self).setFilterWildcard(
            unicode(filter_wildcard))


class MyModel(TreeModel):

    def __init__(self, root, parent=None):
        QAbstractItemModel.__init__(self, parent)
        self.rootItem = root

    def insert_row(self, data, append=True):
        parent = data.parentItem
        parent_index = self.get_model_index(parent)
        row_index = len(parent.childItems)
        self.beginInsertRows(parent_index, row_index, 1)
        if append:
            parent.appendChild(data)
        self.endInsertRows()
        return True

    def get_model_index(self, data):
        indexes = []
        while data.parentItem is not None:
            indexes.append(data.parentItem.childItems.index(data))
            data = data.parentItem
        model_index = self.createIndex(0, 0, self.rootItem)
        for i in reversed(indexes):
            model_index = self.index(i, 0, model_index)
        return model_index

    def data(self, index, role=Qt.DisplayRole):
        if not index.isValid():
            return None
        item = index.internalPointer()
        if role == Qt.DisplayRole:
            return item.data(index.column())


class MyWidget(QWidget):

    phonetic_alphabet = ['Alpha', 'Bravo', 'Charlie', 'Delta', 'Echo', 'Foxtrot',
        'Golf', 'Hotel', 'India', 'Juliet', 'Kilo', 'Lima', 'Mike', 'November',
        'Oscar', 'Papa', 'Quebec', 'Romeo', 'Sierra', 'Tango', 'Uniform', 'Victor',
        'Whiskey', 'X-Ray', 'Yankee', 'Zulu']


    def start_structure(self):
        self.root = MyTreeItem(['Root'])
        self.alphabet = MyTreeItem(['Alphabet'], parent=self.root)

        alpha = self.available.pop(0)
        bravo = self.available.pop(0)
        charlie = self.available.pop(0)
        MyTreeItem([alpha], parent=self.alphabet)
        MyTreeItem([bravo], parent=self.alphabet)
        MyTreeItem([charlie], parent=self.alphabet)


    def __init__(self):
        QWidget.__init__(self)
        self.available = self.phonetic_alphabet[:]
        layout = QVBoxLayout()
        self.ed_filter = QLineEdit()
        self.ed_filter.textChanged.connect(self.on_ed_filter_changed)
        layout.addWidget(self.ed_filter)
        self.tree = QTreeView()
        layout.addWidget(self.tree)
        self.btn = QPushButton('Go')
        self.btn.clicked.connect(self.on_btn_clicked)
        layout.addWidget(self.btn)
        self.btn_reset = QPushButton('Reset')
        self.btn_reset.clicked.connect(self.on_btn_reset_clicked)
        layout.addWidget(self.btn_reset)
        self.setLayout(layout)
        self.start_structure()

        model = MyModel(self.root)

        # This doesnt work
        proxymodel = MyProxyModel()
        proxymodel.setSourceModel(model)
        self.tree.setModel(proxymodel)

        # This works
#        self.tree.setModel(model)


    def on_ed_filter_changed(self, text):
        text = unicode(text).lower()
        self.tree.model().setFilterWildcard(text)


    def on_btn_clicked(self):
        x = self.available.pop(0)
        data = TreeItem([x], parent=self.alphabet)
        self.tree.model().sourceModel().insert_row(data) # This doesnt work
#        self.tree.model().insert_row(data) # This works
        print 'clicked'


    def on_btn_reset_clicked(self):
        self.tree.model().reset()


if __name__ == '__main__':
    app = QApplication([])
    widget = MyWidget()
    widget.show()

    app.exec_()