Merge bitcoin#18062: gui: Fix unintialized WalletView::progressDialog

acf8abc gui: Fix unintialized WalletView::progressDialog (João Barbosa)

Pull request description:

  bitcoin#17911 shows that it's possible to read the unintialized `progressDialog` in https://github.com/bitcoin/bitcoin/blob/f32564f0a73c5ad1a107dd112e40516f39d1a51e/src/qt/walletview.cpp#L296-L297.

  And the debugger shows
  ```
  (gdb) bt
  #0  0x0000555556687c60 in QProgressDialog::wasCanceled() const ()
  #1  0x000055555572989f in WalletView::showProgress (this=0x5555577d7a70,
      title=..., nProgress=1) at qt/walletview.cpp:322
  ```

  Closes bitcoin#17911.

ACKs for top commit:
  hebasto:
    ACK acf8abc, I have reviewed the code and it looks OK, I agree it can be merged.
  elichai:
    utACK acf8abc
  kristapsk:
    ACK acf8abc
  MarcoFalke:
    ACK acf8abc

Tree-SHA512: f5e6d873192d08d1a572e66e17c2e06d1ce27d01aa196b2a7ed591008641295bb02cda8ac90919ff2d2fc778316c2e143f8d36599e0d377779758853dfaf0a31

src/qt/walletview.h

@@ -66,7 +66,7 @@ class WalletView : public QStackedWidget
 
     TransactionView *transactionView;
 
-    QProgressDialog *progressDialog;
+    QProgressDialog* progressDialog{nullptr};
     const PlatformStyle *platformStyle;
 
 public Q_SLOTS: