Merge #16135: gui: Set progressDialog to nullptr

d2ae6be80f gui: Set progressDialog to nullptr (João Barbosa)

Pull request description:

  If a progress notification `> 0` arrives immediately after notification `= 100` then `progressDialog` is a dangling pointer.

  Potential fix for #16134.

ACKs for commit d2ae6b:
  hebasto:
    utACK d2ae6be80f
  fanquake:
    tACK https://github.com/bitcoin/bitcoin/pull/16135/commits/d2ae6be80f6a0156021bf8c9b9d17cd4966ddffc

Tree-SHA512: 300ddde2f27c494b19a5bd4085400d0f5a1d4980fe8cc3c07bfebb037efc35f777215ff1a095eeb16658407e11f04456137393e88a12fdd767b7aac5f12eab5e

src/qt/bitcoingui.cpp

@@ -1341,6 +1341,7 @@ void BitcoinGUI::showProgress(const QString &title, int nProgress)
         if (progressDialog) {
             progressDialog->close();
             progressDialog->deleteLater();
+            progressDialog = nullptr;
         }
     } else if (progressDialog) {
         progressDialog->setValue(nProgress);

src/qt/walletview.cpp

@@ -316,6 +316,7 @@ void WalletView::showProgress(const QString &title, int nProgress)
         if (progressDialog) {
             progressDialog->close();
             progressDialog->deleteLater();
+            progressDialog = nullptr;
         }
     } else if (progressDialog) {
         if (progressDialog->wasCanceled()) {