[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/transactionview.cpp

@@ -83,14 +83,12 @@ TransactionView::TransactionView(QWidget *parent) :
 
     addressWidget = new QLineEdit(this);
 #if QT_VERSION >= 0x040700
-    /* Do not move this to the XML file, Qt before 4.7 will choke on it */
     addressWidget->setPlaceholderText(tr("Enter address or label to search"));
 #endif
     hlayout->addWidget(addressWidget);
 
     amountWidget = new QLineEdit(this);
 #if QT_VERSION >= 0x040700
-    /* Do not move this to the XML file, Qt before 4.7 will choke on it */
     amountWidget->setPlaceholderText(tr("Min amount"));
 #endif
 #ifdef Q_OS_MAC
@@ -355,10 +353,10 @@ void TransactionView::editLabel()
             // Determine type of address, launch appropriate editor dialog type
             QString type = modelIdx.data(AddressTableModel::TypeRole).toString();
 
-            EditAddressDialog dlg(type==AddressTableModel::Receive
-                                         ? EditAddressDialog::EditReceivingAddress
-                                         : EditAddressDialog::EditSendingAddress,
-                                  this);
+            EditAddressDialog dlg(
+                type == AddressTableModel::Receive
+                ? EditAddressDialog::EditReceivingAddress
+                : EditAddressDialog::EditSendingAddress, this);
             dlg.setModel(addressBook);
             dlg.loadRow(idx);
             dlg.exec();
@@ -367,7 +365,7 @@ void TransactionView::editLabel()
         {
             // Add sending address
             EditAddressDialog dlg(EditAddressDialog::NewSendingAddress,
-                                  this);
+                this);
             dlg.setModel(addressBook);
             dlg.setAddress(address);
             dlg.exec();