Segfault after export to DiveShare

[Sergey Starosek]

Steps to reproduce:

- start subsurface
- select one dive and export it to DiveShare
- close Subsurface

Program received signal SIGSEGV, Segmentation fault.
0x0000000001810b80 in ?? ()
(gdb) bt
#0  0x0000000001810b80 in ?? ()
#1  0x000000000056a464 in DiveShareExportDialog::~DiveShareExportDialog() ()
#2  0x000000000056a489 in DiveShareExportDialog::~DiveShareExportDialog() ()
#3  0x00007ffff47eac9c in QObjectPrivate::deleteChildren() () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#4  0x00007ffff590abf9 in QWidget::~QWidget() () from /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#5  0x00000000004bbe29 in MainWindow::~MainWindow() ()
#6  0x0000000000472e26 in exit_ui() ()
#7  0x00000000004391a9 in main ()

Sergey