This fix was found thanks to a user provided memory dump. We don't know
why the layer can be nullptr, but there are some conditions preventing
this situation already in other places of the StandbyState::onMouseDown()
member function.
There are some bug report about problems when the window is minimized.
I was able to reproduce this bug minimizing the window, hiding the dock
(using the system settings), showing the dock again, and trying to
restore the window.
Undoers were little objects to swap/revert an action. They didn't
execute the action itself, they just revert its previous state. Now
undoers were replaced with cmds: A cmd is an object that
executes/undoes/redoes just one action.
Changes:
* Remove old undo library and app/objects_container_impl.cpp
(now we use the doc::ObjectId directly to store undo info)
* Remove all Undoers from app/undoers/
* Replace DocumentApi impl with little Cmds in app/cmd/, these
cmds handle execute/undo/redo of each action at the logic layer
* Remove doc::Dirty object
* Remove doc::Settings: all undo configuration is in the app side
* Move undo options from app:ISettings to app::Preferences
* Rename UndoTransaction to Transaction
* Create a CmdSequence to store a sequence of Cmds (as now the new
undo library doesn't support open/close groups)
* Add doc::get<T>(ObjectId) function to get any kind of object
from the doc library by its ID
* Add Cel::document() and Sprite::document() members
* Add Sprite::cels(frame_t) to get all cels in the given frame
* Add Layer::displaceFrames() member function
* Move the "allow non-linear history" flag from undo2::UndoHistory
to app::DocumentUndo
Changes:
* Add doc::ImageRef to count references to the same image between Cels
(at this moment we cannot generate linked cels anyway)
* Remove doc:Stock class and doc::Sprite::m_stock member variable
* Remove app::undoers::Add/RemoveImage
* Add doc::SubObjectsIO and app::undoers::ObjectIO to
replace doc::LayerSubObjectsSerializer
