[Bug 1705704] [NEW] Amarok APG: crashes if checkpoint is unassigned in CheckpointConstraint

[PascalC]

Public bug reported:

amarok allows me to generate an incomplete model of APL using an
incomplete "checkpoint" constraint.

to reproduce start with for example this APL model :
pascal at minipc:~$ cat Musique/classiques3.xml 
<?xml version="1.0" encoding="UTF-8"?>
<playlistgenerator>
  <generatorpreset title="classiques3">
    <constrainttree>
      <group matchtype="all">
        <constraint field="genre" comparison="3" invert="false" type="TagMatch" value="Classical" strictness="0.1"/>
      </group>
    </constrainttree>
  </generatorpreset>
</playlistgenerator>

then just 
add a "checkpoint" constraint to the constrainttree (see screenshot)
but do not select anything as checkpoint parameters, 
just adjust strictness of checkpoint to exact
click OK

the new model is saved

when editing this model you see the 2 constraints are there as in the
screenshot but when exporting this model to a file the "checkpoint"
constraint is absent !

now try to execute this model ==> amarok crashes

running in debug mode gives this upon executing this incomplete model :

pascal at minipc:~$ amarok: [APG::ConstraintSolver] New ConstraintSolver with serial number 967666005 
amarok: [APG::ConstraintNode] new constraint node at 0x31de3f90 with parent at 0x2f6023f0 
amarok: [APG::ConstraintSolver] Domain has 4376 tracks 
amarok: [APG::ConstraintSolver] Running ConstraintSolver 967666005 
amarok: Fatal IO error: client killed
Unable to start Dr. Konqi
Not forwarding the crash to Apport.

ProblemType: Bug
DistroRelease: Ubuntu 17.04
Package: amarok 2:2.8.0-0ubuntu9
ProcVersionSignature: Ubuntu 4.10.0-26.30-lowlatency 4.10.17
Uname: Linux 4.10.0-26-lowlatency x86_64
ApportVersion: 2.20.4-0ubuntu4.5
Architecture: amd64
CurrentDesktop: KDE
Date: Fri Jul 21 15:18:23 2017
InstallationDate: Installed on 2016-05-14 (432 days ago)
InstallationMedia: Kubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
SourcePackage: amarok
UpgradeStatus: No upgrade log present (probably fresh install)

** Affects: amarok
     Importance: Unknown
         Status: Unknown

** Affects: amarok (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug third-party-packages zesty

** Bug watch added: KDE Bug Tracking System #382456
   https://bugs.kde.org/show_bug.cgi?id=382456

** Also affects: amarok via
   https://bugs.kde.org/show_bug.cgi?id=382456
   Importance: Unknown
       Status: Unknown

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Title:
  Amarok APG: crashes if checkpoint is unassigned in
  CheckpointConstraint

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