add empty constructor.

src/musredit_qt5/mupp/var/include/PVarHandler.h

@@ -41,6 +41,7 @@
 
 class PVarHandler {
   public:
+    PVarHandler();
     PVarHandler(PmuppCollection *coll, std::string parse_str, std::string var_name);
 
     bool isValid() { return fIsValid; }

src/musredit_qt5/mupp/var/src/PVarHandler.cpp

@@ -36,6 +36,16 @@
 #include "PStatement.hpp"
 #include "PProgram.hpp"
 
+//--------------------------------------------------------------------------
+/**
+ * @brief PVarHandler::PVarHandler
+ */
+PVarHandler::PVarHandler() :
+  fColl(nullptr), fParseStr(""), fVarName(""), fIsValid(false)
+{
+  // nothing to be done here
+}
+
 //--------------------------------------------------------------------------
 /**
  * @brief PVarHandler::PVarHandler