Crash when clicking on app dock icon

Latest master.
macOS 10.15.4 (19E242d)

Reproducibility: always

Steps:
Launch app and keep clicking on the BitcoinABC dock icon.

What happened:
Crash.

```
Crashed Thread: 0 Dispatch queue: com.apple.main-thread

Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x0000000000000010
Exception Note: EXC_CORPSE_NOTIFY

Termination Signal: Segmentation fault: 11
Termination Reason: Namespace SIGNAL, Code 0xb
Terminating Process: exc handler [24435]

VM Regions Near 0x10:
-->
__TEXT 00000001092a5000-000000010abc4000 [ 25.1M] r-x/r-x SM=COW /Users/USER/*/BitcoinABC-Qt.app/Contents/MacOS/BitcoinABC-Qt

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 org.bitcoinabc.BitcoinABC-Qt 0x0000000109377f1e GUIUtil::TableViewLastColumnResizingFixer::disconnectViewHeadersSignals() + 30 (guiutil.cpp:456)
1 org.bitcoinabc.BitcoinABC-Qt 0x000000010937857d GUIUtil::TableViewLastColumnResizingFixer::stretchColumnWidth(int) + 45 (guiutil.cpp:516)
2 org.bitcoinabc.BitcoinABC-Qt 0x000000010957c15a TransactionView::resizeEvent(QResizeEvent*) + 74 (transactionview.cpp:682)
3 org.qt-project.QtWidgets 0x000000010d8845b3 QWidget::event(QEvent*) + 2071
4 org.qt-project.QtWidgets 0x000000010d85c70a QApplicationPrivate::notify_helper(QObject*, QEvent*) + 218
5 org.qt-project.QtWidgets 0x000000010d85d5d0 QApplication::notify(QObject*, QEvent*) + 546
6 org.qt-project.QtCore 0x000000010e30dc76 QCoreApplication::notifyInternal2(QObject*, QEvent*) + 138
7 org.qt-project.QtWidgets 0x000000010d87f262 QWidgetPrivate::sendPendingMoveAndResizeEvents(bool, bool) + 208
8 org.qt-project.QtWidgets 0x000000010d8831f5 QWidgetPrivate::show_helper() + 41
9 org.qt-project.QtWidgets 0x000000010d883c0d QWidgetPrivate::setVisible(bool) + 647
10 org.qt-project.QtWidgets 0x000000010d88348b QWidgetPrivate::showChildren(bool) + 221
11 org.qt-project.QtWidgets 0x000000010d88320d QWidgetPrivate::show_helper() + 65
12 org.qt-project.QtWidgets 0x000000010d883c0d QWidgetPrivate::setVisible(bool) + 647
13 org.qt-project.QtWidgets 0x000000010d88348b QWidgetPrivate::showChildren(bool) + 221
14 org.qt-project.QtWidgets 0x000000010d88320d QWidgetPrivate::show_helper() + 65
15 org.qt-project.QtWidgets 0x000000010d883c0d QWidgetPrivate::setVisible(bool) + 647
16 org.qt-project.QtWidgets 0x000000010d88348b QWidgetPrivate::showChildren(bool) + 221
17 org.qt-project.QtWidgets 0x000000010d88320d QWidgetPrivate::show_helper() + 65
18 org.qt-project.QtWidgets 0x000000010d883c0d QWidgetPrivate::setVisible(bool) + 647
19 org.qt-project.QtWidgets 0x000000010d88348b QWidgetPrivate::showChildren(bool) + 221
20 org.qt-project.QtWidgets 0x000000010d88320d QWidgetPrivate::show_helper() + 65
21 org.qt-project.QtWidgets 0x000000010d883c0d QWidgetPrivate::setVisible(bool) + 647
22 org.bitcoinabc.BitcoinABC-Qt 0x000000010933337d BitcoinGUI::macosDockIconActivated() + 45 (bitcoingui.cpp:772)
23 org.bitcoinabc.BitcoinABC-Qt 0x0000000109311da2 QtPrivate::FunctorCall, QtPrivate::List<>, void, void (BitcoinGUI::*)()>::call(void (BitcoinGUI::*)(), BitcoinGUI*, void**) + 146 (qobjectdefs_impl.h:152)
24 org.bitcoinabc.BitcoinABC-Qt 0x0000000109311c56 void QtPrivate::FunctionPointer::call, void>(void (BitcoinGUI::*)(), BitcoinGUI*, void**) + 102 (qobjectdefs_impl.h:185)
25 org.bitcoinabc.BitcoinABC-Qt 0x0000000109311aa3 QtPrivate::QSlotObject, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) + 179 (qobjectdefs_impl.h:415)
26 org.qt-project.QtCore 0x000000010e32c2b8 0x10e1db000 + 1381048
27 org.bitcoinabc.BitcoinABC-Qt 0x00000001092cf013 MacDockIconHandler::dockIconClicked() + 51
28 org.bitcoinabc.BitcoinABC-Qt 0x000000010945121a dockClickHandler(objc_object*, objc_selector*, ...) + 42 (macdockiconhandler.mm:16)
29 com.apple.AppKit 0x00007fff2d7c0ff8 -[NSApplication(NSAppleEventHandling) _handleAEReopen:] + 279
30 com.apple.AppKit 0x00007fff2d5dfb98 -[NSApplication(NSAppleEventHandling) _handleCoreEvent:withReplyEvent:] + 688
31 com.apple.Foundation 0x00007fff329afb06 -[NSAppleEventManager dispatchRawAppleEvent:withRawReply:handlerRefCon:] + 308
32 com.apple.Foundation 0x00007fff329af970 _NSAppleEventManagerGenericHandler + 98
33 com.apple.AE 0x00007fff316bb203 0x7fff316ae000 + 53763
34 com.apple.AE 0x00007fff316ba929 0x7fff316ae000 + 51497
35 com.apple.AE 0x00007fff316b2bd7 aeProcessAppleEvent + 449
36 com.apple.HIToolbox 0x00007fff2ef8c92a AEProcessAppleEvent + 54
37 com.apple.AppKit 0x00007fff2d5d9f31 _DPSNextEvent + 1547
38 com.apple.AppKit 0x00007fff2d5d84e0 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 1352
39 libqcocoa.dylib 0x0000000112eeb160 0x112ebe000 + 184672
40 org.bitcoinabc.BitcoinABC-Qt 0x00000001095016dc SendCoinsDialog::addEntry() + 876 (sendcoinsdialog.cpp:461)
41 org.bitcoinabc.BitcoinABC-Qt 0x00000001094f70a6 SendCoinsDialog::SendCoinsDialog(PlatformStyle const*, WalletModel*, QWidget*) + 1350 (sendcoinsdialog.cpp:56)
42 org.bitcoinabc.BitcoinABC-Qt 0x00000001095040fb SendCoinsDialog::SendCoinsDialog(PlatformStyle const*, WalletModel*, QWidget*) + 59 (sendcoinsdialog.cpp:39)
43 org.bitcoinabc.BitcoinABC-Qt 0x00000001095d1134 WalletView::WalletView(PlatformStyle const*, WalletModel*, QWidget*) + 1652 (walletview.cpp:56)
44 org.bitcoinabc.BitcoinABC-Qt 0x00000001095d26cb WalletView::WalletView(PlatformStyle const*, WalletModel*, QWidget*) + 59 (walletview.cpp:35)
45 org.bitcoinabc.BitcoinABC-Qt 0x0000000109593ac8 WalletFrame::addWallet(WalletModel*) + 216 (walletframe.cpp:46)
46 org.bitcoinabc.BitcoinABC-Qt 0x0000000109332940 BitcoinGUI::addWallet(WalletModel*) + 128 (bitcoingui.cpp:632)
47 org.bitcoinabc.BitcoinABC-Qt 0x000000010933266a BitcoinGUI::setWalletController(WalletController*) + 554 (bitcoingui.cpp:621)
48 org.bitcoinabc.BitcoinABC-Qt 0x00000001092f7c4b BitcoinApplication::initializeResult(bool) + 955 (bitcoin.cpp:391)
49 org.bitcoinabc.BitcoinABC-Qt 0x0000000109313c92 QtPrivate::FunctorCall, QtPrivate::List, void, void (BitcoinApplication::*)(bool)>::call(void (BitcoinApplication::*)(bool), BitcoinApplication*, void**) + 162 (qobjectdefs_impl.h:152)
50 org.bitcoinabc.BitcoinABC-Qt 0x0000000109313b76 void QtPrivate::FunctionPointer::call, void>(void (BitcoinApplication::*)(bool), BitcoinApplication*, void**) + 102 (qobjectdefs_impl.h:185)
51 org.bitcoinabc.BitcoinABC-Qt 0x0000000109313a33 QtPrivate::QSlotObject, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) + 179 (qobjectdefs_impl.h:415)
52 org.qt-project.QtCore 0x000000010e327a3d QObject::event(QEvent*) + 693
53 org.qt-project.QtGui 0x000000010dcd5571 QGuiApplication::event(QEvent*) + 163
54 org.qt-project.QtWidgets 0x000000010d85b5ca QApplication::event(QEvent*) + 764
55 org.qt-project.QtWidgets 0x000000010d85c70a QApplicationPrivate::notify_helper(QObject*, QEvent*) + 218
56 org.qt-project.QtWidgets 0x000000010d85d5d0 QApplication::notify(QObject*, QEvent*) + 546
57 org.qt-project.QtCore 0x000000010e30dc76 QCoreApplication::notifyInternal2(QObject*, QEvent*) + 138
58 org.qt-project.QtCore 0x000000010e30e729 QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) + 389
59 libqcocoa.dylib 0x0000000112eebb7e 0x112ebe000 + 187262
60 libqcocoa.dylib 0x0000000112eec12a 0x112ebe000 + 188714
61 com.apple.CoreFoundation 0x00007fff30349552 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
62 com.apple.CoreFoundation 0x00007fff303494f1 __CFRunLoopDoSource0 + 103
63 com.apple.CoreFoundation 0x00007fff3034930b __CFRunLoopDoSources0 + 209
64 com.apple.CoreFoundation 0x00007fff3034803a __CFRunLoopRun + 927
65 com.apple.CoreFoundation 0x00007fff3034763e CFRunLoopRunSpecific + 462
66 com.apple.HIToolbox 0x00007fff2ef7ac0d RunCurrentEventLoopInMode + 292
67 com.apple.HIToolbox 0x00007fff2ef7a925 ReceiveNextEventCommon + 584
68 com.apple.HIToolbox 0x00007fff2ef7a6c9 _BlockUntilNextEventMatchingListInModeWithFilter + 64
69 com.apple.AppKit 0x00007fff2d5d9c99 _DPSNextEvent + 883
70 com.apple.AppKit 0x00007fff2d5d84e0 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 1352
71 com.apple.AppKit 0x00007fff2d5ca1ee -[NSApplication run] + 658
72 libqcocoa.dylib 0x0000000112eeb420 0x112ebe000 + 185376
73 org.qt-project.QtCore 0x000000010e30ba57 QEventLoop::exec(QFlags) + 267
74 org.qt-project.QtCore 0x000000010e30e0cf QCoreApplication::exec() + 123
75 org.bitcoinabc.BitcoinABC-Qt 0x00000001092fb17b GuiMain(int, char**) + 5819 (bitcoin.cpp:729)
76 org.bitcoinabc.BitcoinABC-Qt 0x00000001092acbe0 main + 48 (main.cpp:19)
77 libdyld.dylib 0x00007fff6a284cc9 start + 1
```

Expected result:
BitcoinABC does not crash.