Unable to compile

It seems to me that earlier I have compiled this version succesfully. Something changed in the environment after a routine upgrade?
djvuwidget.cpp: In constructor ‘DjVuWidget::DjVuWidget(QWidget*)’:
djvuwidget.cpp:23:15: error: no match for ‘operator=’ (operand types are ‘QSharedPointer<QDjVuNetDocument>’ and ‘std::nullptr_t’)
  m_document = nullptr;
               ^~~~~~~
In file included from /usr/include/x86_64-linux-gnu/qt5/QtCore/qsharedpointer.h:48:0,
                 from /usr/include/x86_64-linux-gnu/qt5/QtCore/qdebug.h:54,
                 from /usr/include/x86_64-linux-gnu/qt5/QtCore/QtCore:29,
                 from djvulink.h:18,
                 from djvuwidget.h:18,
                 from djvuwidget.cpp:15:
/usr/include/x86_64-linux-gnu/qt5/QtCore/qsharedpointer_impl.h:327:21: note: candidate: QSharedPointer<T>& QSharedPointer<T>::operator=(const QSharedPointer<T>&) [with T = QDjVuNetDocument]
     QSharedPointer &operator=(const QSharedPointer &other) Q_DECL_NOTHROW
                     ^~~~~~~~
/usr/include/x86_64-linux-gnu/qt5/QtCore/qsharedpointer_impl.h:327:21: note:   no known conversion for argument 1 from ‘std::nullptr_t’ to ‘const QSharedPointer<QDjVuNetDocument>&’
/usr/include/x86_64-linux-gnu/qt5/QtCore/qsharedpointer_impl.h:340:21: note: candidate: QSharedPointer<T>& QSharedPointer<T>::operator=(QSharedPointer<T>&&) [with T = QDjVuNetDocument]
     QSharedPointer &operator=(QSharedPointer &&other) Q_DECL_NOTHROW
                     ^~~~~~~~
/usr/include/x86_64-linux-gnu/qt5/QtCore/qsharedpointer_impl.h:340:21: note:   no known conversion for argument 1 from ‘std::nullptr_t’ to ‘QSharedPointer<QDjVuNetDocument>&&’
/usr/include/x86_64-linux-gnu/qt5/QtCore/qsharedpointer_impl.h:356:21: note: candidate: template<class X> QSharedPointer<T>& QSharedPointer<T>::operator=(QSharedPointer<X>&&) [with X = X; T = QDjVuNetDocument]
     QSharedPointer &operator=(QSharedPointer<X> &&other) Q_DECL_NOTHROW
                     ^~~~~~~~
/usr/include/x86_64-linux-gnu/qt5/QtCore/qsharedpointer_impl.h:356:21: note:   template argument deduction/substitution failed:
djvuwidget.cpp:23:15: note:   mismatched types ‘QSharedPointer<T>’ and ‘std::nullptr_t’
  m_document = nullptr;
               ^~~~~~~
In file included from /usr/include/x86_64-linux-gnu/qt5/QtCore/qsharedpointer.h:48:0,
                 from /usr/include/x86_64-linux-gnu/qt5/QtCore/qdebug.h:54,
                 from /usr/include/x86_64-linux-gnu/qt5/QtCore/QtCore:29,
                 from djvulink.h:18,
                 from djvuwidget.h:18,
                 from djvuwidget.cpp:15:
/usr/include/x86_64-linux-gnu/qt5/QtCore/qsharedpointer_impl.h:370:28: note: candidate: template<class X> QSharedPointer<T>& QSharedPointer<T>::operator=(const QSharedPointer<X>&) [with X = X; T = QDjVuNetDocument]
     inline QSharedPointer &operator=(const QSharedPointer<X> &other)
                            ^~~~~~~~
/usr/include/x86_64-linux-gnu/qt5/QtCore/qsharedpointer_impl.h:370:28: note:   template argument deduction/substitution failed:
djvuwidget.cpp:23:15: note:   mismatched types ‘const QSharedPointer<T>’ and ‘std::nullptr_t’
  m_document = nullptr;
               ^~~~~~~
In file included from /usr/include/x86_64-linux-gnu/qt5/QtCore/qsharedpointer.h:48:0,
                 from /usr/include/x86_64-linux-gnu/qt5/QtCore/qdebug.h:54,
                 from /usr/include/x86_64-linux-gnu/qt5/QtCore/QtCore:29,
                 from djvulink.h:18,
                 from djvuwidget.h:18,
                 from djvuwidget.cpp:15:
/usr/include/x86_64-linux-gnu/qt5/QtCore/qsharedpointer_impl.h:382:31: note: candidate: template<class X> QSharedPointer<T>& QSharedPointer<T>::operator=(const QWeakPointer<X>&) [with X = X; T = QDjVuNetDocument]
     inline QSharedPointer<T> &operator=(const QWeakPointer<X> &other)
                               ^~~~~~~~
/usr/include/x86_64-linux-gnu/qt5/QtCore/qsharedpointer_impl.h:382:31: note:   template argument deduction/substitution failed:
djvuwidget.cpp:23:15: note:   mismatched types ‘const QWeakPointer<T>’ and ‘std::nullptr_t’
  m_document = nullptr;
               ^~~~~~~
Makefile:812: recipe for target 'djvuwidget.o' failed
make[2]: *** [djvuwidget.o] Error 1
make[2]: Leaving directory '/home/jsbien/djview4poliqarp/djview-poliqarp/src'
debian/rules:8: recipe for target 'override_dh_auto_build' failed
make[1]: *** [override_dh_auto_build] Error 2
make[1]: Leaving directory '/home/jsbien/djview4poliqarp/djview-poliqarp'
debian/rules:28: recipe for target 'build' failed
make: *** [build] Error 2
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