Shape builder crashes
#### Summary:
The Shape Builder (sometimes) crashes on completion.
#### Steps to reproduce:
* Open Inkscape.
* Draw two overlapping rectangles.
* Press 'x' (activate the shape builder).
* Drag through all 3 regions.
* Press enter (complete shape builder).
{width=800 height=486}
#### What happened?
Crash
```
7 Inkscape::Application::crash_handler(int) inkscape.cpp 441
9 Inkscape::UI::Tools::InteractiveBooleansTool::hide_selected_objects(bool) booleans-tool.cpp 81
10 Inkscape::UI::Tools::InteractiveBooleansTool::~InteractiveBooleansTool() booleans-tool.cpp 62
11 Inkscape::UI::Tools::InteractiveBooleansTool::~InteractiveBooleansTool() booleans-tool.cpp 64
12 void delete_then_null desktop.cpp 78
13 SPDesktop::setTool(std::stringconst&) desktop.cpp 323
14 tool_switch(Glib::ustring const&, InkscapeWindow *) actions-tools.cpp 170
15 set_active_tool(InkscapeWindow *, Glib::ustring const&) actions-tools.cpp 74
16 set_active_tool(SPDesktop *, Glib::ustring const&) actions-tools.cpp 268
17 Inkscape::UI::Tools::InteractiveBooleansTool::shape_commit() booleans-tool.cpp 140
18 Inkscape::UI::Tools::InteractiveBooleansTool::event_key_press_handler(Inkscape::KeyPressEvent const&) booleans-tool.cpp 275
19 void Inkscape::inspect_event booleans-tool.cpp 165
20 Inkscape::UI::Tools::InteractiveBooleansTool::root_handler(Inkscape::CanvasEvent const&) booleans-tool.cpp 157
```
from the vector of dangling pointers added in inkscape!7511.
#### Version info
1.5-dev master AppImage (inkscape@d7a538637fa8b5fd74c5ab60ed320501868e9af8)
issue