Fixed up a bunch of compiler warnings.

There should be no functional difference, just less noise when compiling

connections/canconnectionmodel.cpp

@@ -87,11 +87,11 @@ QVariant CANConnectionModel::data(const QModelIndex &index, int role) const
                         case CANCon::GVRET_SERIAL: return "GVRET";
                         default: {}
                     }
-                else qDebug() << "Tried to show connection type but connection was NULL";
+                else qDebug() << "Tried to show connection type but connection was nullptr";
                 break;
             case Column::Port:
                 if (conn_p) return conn_p->getPort();
-                else qDebug() << "Tried to show connection port but connection was NULL";
+                else qDebug() << "Tried to show connection port but connection was nullptr";
                 break;
             case Column::Subtype:
                 return conn_p->getDriver();
@@ -138,7 +138,7 @@ void CANConnectionModel::replace(int idx , CANConnection* pConn_p)
 CANConnection* CANConnectionModel::getAtIdx(int pIdx) const
 {
     if (pIdx < 0)
-        return NULL;
+        return nullptr;
 
     QList<CANConnection*>& conns = CANConManager::getInstance()->getConnections();