[Qt] make most Windows appear centered on main GUI

- remove default value for parent variable in dialogs, which should appear
  centered over main GUI and pass appropriate value
- add some addition NULL pointer checks

src/qt/bitcoingui.cpp

@@ -10,12 +10,12 @@
 #include "guiconstants.h"
 #include "guiutil.h"
 #include "notificator.h"
+#include "openuridialog.h"
 #include "optionsdialog.h"
 #include "optionsmodel.h"
 #include "rpcconsole.h"
 #include "walletframe.h"
 #include "walletmodel.h"
-#include "openuridialog.h"
 
 #ifdef Q_OS_MAC
 #include "macdockiconhandler.h"
@@ -345,7 +345,7 @@ void BitcoinGUI::setClientModel(ClientModel *clientModel)
         setNumBlocks(clientModel->getNumBlocks(), clientModel->getNumBlocksOfPeers());
         connect(clientModel, SIGNAL(numBlocksChanged(int,int)), this, SLOT(setNumBlocks(int,int)));
 
-        // Receive and report messages from network/worker thread
+        // Receive and report messages from client model
         connect(clientModel, SIGNAL(message(QString,QString,unsigned int)), this, SLOT(message(QString,QString,unsigned int)));
 
         rpcConsole->setClientModel(clientModel);
@@ -460,21 +460,25 @@ void BitcoinGUI::optionsClicked()
 {
     if(!clientModel || !clientModel->getOptionsModel())
         return;
-    OptionsDialog dlg;
+
+    OptionsDialog dlg(this);
     dlg.setModel(clientModel->getOptionsModel());
     dlg.exec();
 }
 
 void BitcoinGUI::aboutClicked()
 {
-    AboutDialog dlg;
+    if(!clientModel)
+        return;
+
+    AboutDialog dlg(this);
     dlg.setModel(clientModel);
     dlg.exec();
 }
 
 void BitcoinGUI::openClicked()
 {
-    OpenURIDialog dlg;
+    OpenURIDialog dlg(this);
     if(dlg.exec())
     {
         emit receivedURI(dlg.getURI());