Segfault-generating PyQt5 code

#!/usr/bin/env python3

# This is based on Baz Walter's example code for the QsciLexerCustom class.
# The added code producing a segmentation fault is marked below in the
# __init__ method of the MainWindow class.

import sys
from PyQt5 import QtCore, QtGui, QtWidgets, Qsci


class MainWindow(QtWidgets.QMainWindow):
     def __init__(self):
         QtWidgets.QMainWindow.__init__(self)
         self.setWindowTitle('Custom Lexer Example')
         self.setGeometry(QtCore.QRect(50,200,400,400))
         self.editor = Qsci.QsciScintilla(self)
         self.editor.setUtf8(True)
         self.editor.setMarginWidth(2, 15)
         self.editor.setFolding(True)

         # The following three lines of code cause this program to generate a
         # segfault for me upon closing the main window. The original code it
         # replaces is:
         #
         # self.setCentralWidget(self.editor)
         self.tabWidget = QtWidgets.QTabWidget()
         self.tabWidget.addTab(self.editor, 'Untitled')
         self.setCentralWidget(self.tabWidget)

         self.lexer = CustomLexer(self.editor)
         self.editor.setLexer(self.lexer)
         self.editor.setText('\n# sample source\n\nfoo = 1\nbar = 2\n')


class CustomLexer(Qsci.QsciLexerCustom):
     def __init__(self, parent):
         Qsci.QsciLexerCustom.__init__(self, parent)
         self._styles = {
             0: 'Default',
             1: 'Comment',
             2: 'Key',
             3: 'Assignment',
             4: 'Value',
             }
         for key in self._styles:
             setattr(self, self._styles[key], key)

     def description(self, style):
         return self._styles.get(style, '')

     def defaultColor(self, style):
         if style == self.Default:
             return QtGui.QColor('#000000')
         elif style == self.Comment:
             return QtGui.QColor('#C0C0C0')
         elif style == self.Key:
             return QtGui.QColor('#0000CC')
         elif style == self.Assignment:
             return QtGui.QColor('#CC0000')
         elif style == self.Value:
             return QtGui.QColor('#00CC00')
         return Qsci.QsciLexerCustom.defaultColor(self, style)

     def styleText(self, start, end):
         editor = self.editor()
         if editor is None:
             return

         # scintilla works with encoded bytes, not decoded characters.
         # this matters if the source contains non-ascii characters and
         # a multi-byte encoding is used (e.g. utf-8)
         source = b''
         if end > editor.length():
             end = editor.length()
         if end > start:
             source = editor.text().encode('utf-8')[start:end]
         if not source:
             return

         # the line index will also be needed to implement folding
         index = editor.SendScintilla(editor.SCI_LINEFROMPOSITION, start)
         if index > 0:
             # the previous state may be needed for multi-line styling
             pos = editor.SendScintilla(
                       editor.SCI_GETLINEENDPOSITION, index - 1)
             state = editor.SendScintilla(editor.SCI_GETSTYLEAT, pos)
         else:
             state = self.Default

         set_style = self.setStyling
         self.startStyling(start, 0x1f)

         # scintilla always asks to style whole lines
         for line in source.splitlines(True):
             length = len(line)
             if line.startswith(b'#'):
                 state = self.Comment
             else:
                 # the following will style lines like "x = 0"
                 pos = line.find(b'=')
                 if pos > 0:
                     set_style(pos, self.Key)
                     set_style(1, self.Assignment)
                     length = length - pos - 1
                     state = self.Value
                 else:
                     state = self.Default
             set_style(length, state)
             # folding implementation goes here
             index += 1


if __name__ == "__main__":
     app = QtWidgets.QApplication(sys.argv)
     app.lastWindowClosed.connect(app.quit)
     win = MainWindow()
     win.show()
     sys.exit(app.exec_())