[libcalamares] Enforce singleton-ness of CalamaresPython::Helper

- unset instance pointer on destruction
 - make constructor private, and the instance accessor
   should create an instance if there isn't one.

src/libcalamares/JobQueue.cpp

@@ -41,9 +41,6 @@ public:
         , m_queue( queue )
         , m_jobIndex( 0 )
     {
-#ifdef WITH_PYTHON
-        new CalamaresPython::Helper( this );
-#endif
     }
 
     void setJobs( const JobList& jobs )

src/libcalamares/PythonHelper.cpp

@@ -231,7 +231,9 @@ Helper::Helper( QObject* parent )
 }
 
 Helper::~Helper()
-{}
+{
+    s_instance = nullptr;
+}
 
 
 boost::python::dict

src/libcalamares/PythonHelper.h

@@ -48,7 +48,6 @@ class Helper : public QObject
 {
     Q_OBJECT
 public:
-    explicit Helper( QObject* parent = nullptr );
     virtual ~Helper();
 
     boost::python::dict createCleanNamespace();
@@ -57,6 +56,7 @@ public:
 
 private:
     friend Helper* Calamares::PythonJob::helper();
+    explicit Helper( QObject* parent = nullptr );
     static Helper* s_instance;
 
     boost::python::object m_mainModule;

src/libcalamares/PythonJob.cpp

@@ -381,8 +381,10 @@ PythonJob::emitProgress( qreal progressValue )
 CalamaresPython::Helper*
 PythonJob::helper()
 {
-    return CalamaresPython::Helper::s_instance;
-
+    auto ptr = CalamaresPython::Helper::s_instance;
+    if (!ptr)
+        ptr = new CalamaresPython::Helper;
+    return ptr;
 }