Keep the default net class object per board, not static

Fixes: lp:1711491
* https://bugs.launchpad.net/kicad/+bug/1711491

pcbnew/class_board.h

@@ -188,7 +188,6 @@ private:
 
     int                     m_fileFormatVersionAtLoad;  ///< the version loaded from the file
 
-    NETINFO_LIST            m_NetInfo;              ///< net info list (name, design constraints ..
     std::shared_ptr<CONNECTIVITY_DATA>      m_connectivity;
 
     BOARD_DESIGN_SETTINGS   m_designSettings;
@@ -197,6 +196,7 @@ private:
     PAGE_INFO               m_paper;
     TITLE_BLOCK             m_titles;               ///< text in lower right of screen and plots
     PCB_PLOT_PARAMS         m_plotOptions;
+    NETINFO_LIST            m_NetInfo;              ///< net info list (name, design constraints ..
 
     /**
      * Function chainMarkedSegments

pcbnew/class_netclass.cpp

@@ -86,6 +86,7 @@ NETCLASS::~NETCLASS()
 
 NETCLASSES::NETCLASSES()
 {
+    m_default = std::make_shared<NETCLASS>( NETCLASS::Default );
 }
 
 

pcbnew/class_netclass.h

@@ -235,6 +235,9 @@ private:
     /// all the NETCLASSes except the default one.
     NETCLASS_MAP    m_NetClasses;
 
+    /// the default NETCLASS
+    NETCLASSPTR     m_default;
+
 public:
     NETCLASSES();
     ~NETCLASSES();
@@ -269,10 +272,9 @@ public:
      * Function GetDefault
      * @return the default net class.
      */
-    NETCLASSPTR& GetDefault() const
+    NETCLASSPTR GetDefault() const
     {
-        static NETCLASSPTR defNetClass = std::make_shared<NETCLASS>( NETCLASS::Default );
-        return defNetClass;
+        return m_default;
     }
 
     /**