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.

(cherry picked from commit commontk/PythonQt@bf07cc8)

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

Author: msmolens

src/PythonQt.h

@@ -554,7 +554,7 @@ 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

@@ -109,7 +109,7 @@ PythonQtObjectPtr::PythonQtObjectPtr(PythonQtSafeObjectPtr &&p) :_object(p.takeO
 
 PythonQtObjectPtr::~PythonQtObjectPtr()
 {
-  Py_XDECREF(_object);
+  if (_object && Py_IsInitialized()) Py_XDECREF(_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();
+  }
 }