Fix up some compile warnings.

mainwindow.cpp

@@ -59,27 +59,27 @@ MainWindow::MainWindow(QWidget *parent) :
     HorzHdr->setStretchLastSection(true); //causes the data column to automatically fill the tableview
     connect(HorzHdr, SIGNAL(sectionClicked(int)), this, SLOT(headerClicked(int)));
 
-    graphingWindow = NULL;
-    frameInfoWindow = NULL;
-    playbackWindow = NULL;
-    flowViewWindow = NULL;
-    frameSenderWindow = NULL;
-    dbcMainEditor = NULL;
-    comparatorWindow = NULL;
-    settingsDialog = NULL;
-    firmwareUploaderWindow = NULL;
-    discreteStateWindow = NULL;
-    connectionWindow = NULL;
-    scriptingWindow = NULL;
-    rangeWindow = NULL;
-    dbcFileWindow = NULL;
-    fuzzingWindow = NULL;
-    udsScanWindow = NULL;
-    motorctrlConfigWindow = NULL;
-    isoWindow = NULL;
-    snifferWindow = NULL;
-    bisectWindow = NULL;
-    signalViewerWindow = NULL;
+    graphingWindow = nullptr;
+    frameInfoWindow = nullptr;
+    playbackWindow = nullptr;
+    flowViewWindow = nullptr;
+    frameSenderWindow = nullptr;
+    dbcMainEditor = nullptr;
+    comparatorWindow = nullptr;
+    settingsDialog = nullptr;
+    firmwareUploaderWindow = nullptr;
+    discreteStateWindow = nullptr;
+    connectionWindow = nullptr;
+    scriptingWindow = nullptr;
+    rangeWindow = nullptr;
+    dbcFileWindow = nullptr;
+    fuzzingWindow = nullptr;
+    udsScanWindow = nullptr;
+    motorctrlConfigWindow = nullptr;
+    isoWindow = nullptr;
+    snifferWindow = nullptr;
+    bisectWindow = nullptr;
+    signalViewerWindow = nullptr;
     dbcHandler = DBCHandler::getReference();
     bDirty = false;
     inhibitFilterUpdate = false;