Prevent crashes during and after cleanup

This commit prevents crashes by handling scenarios such as:
(a) object destruction after the Python interpreter has been finalized
(b) object destruction after cleanup, i.e. the singleton no longer exists

Any usage of a Qt enum demonstrates (a).

One example that demonstrates (b) is a QTimer object which is created with a
QApplication parent. PythonQt::cleanup() is called before the QApplication is
completely destroyed, so the code that handles wrapping the QTimer object must
handle the case when the PythonQt singleton no longer exists.

Co-authored-by: Jean-Christophe Fillion-Robin <[email protected]>

Author: msmolens

src/PythonQt.h

@@ -552,7 +552,7 @@ class PYTHONQT_EXPORT PythonQt : public QObject {
   //@{
 
   //! get access to internal data (should not be used on the public API, but is used by some C functions)
-  static PythonQtPrivate* priv() { return _self->_p; }
+  static PythonQtPrivate* priv() { return _self ? _self->_p : NULL; }
 
   //! clear all NotFound entries on all class infos, to ensure that
   //! newly loaded wrappers can add methods even when the object was wrapped by PythonQt before the wrapper was loaded

src/PythonQtObjectPtr.cpp

@@ -99,7 +99,7 @@ PythonQtObjectPtr::PythonQtObjectPtr(PythonQtSafeObjectPtr &&p) :_object(p.takeO
 
 PythonQtObjectPtr::~PythonQtObjectPtr()
 {
-  if (_object) Py_DECREF(_object);
+  if (_object && Py_IsInitialized()) Py_DECREF(_object);
 }
 
 void PythonQtObjectPtr::setNewRef(PyObject* o)

src/PythonQtSignalReceiver.cpp

@@ -178,10 +178,12 @@ PythonQtSignalReceiver::PythonQtSignalReceiver(QObject* obj):PythonQtSignalRecei
 
 PythonQtSignalReceiver::~PythonQtSignalReceiver()
 {
-  // we need the GIL scope here, because the targets keep references to Python objects
-  PYTHONQT_GIL_SCOPE;
-  PythonQt::priv()->removeSignalEmitter(_obj);
-  _targets.clear();
+  if (PythonQt::priv()) {
+    // we need the GIL scope here, because the targets keep references to Python objects
+    PYTHONQT_GIL_SCOPE;
+    PythonQt::priv()->removeSignalEmitter(_obj);
+    _targets.clear();
+  }
 }