Null pointer deference in rules applicator

* thread #7: tid = 0x25050ed, 0x000000010dd7238c libqbscore.1.5.0.dylib`qbs::Internal::RulesEvaluationContext::engine(this=0x0000000000000000) const + 12 at rulesevaluationcontext.h:62, name = 'QThread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
    frame #0: 0x000000010dd7238c libqbscore.1.5.0.dylib`qbs::Internal::RulesEvaluationContext::engine(this=0x0000000000000000) const + 12 at rulesevaluationcontext.h:62
  * frame #1: 0x000000010de336ff libqbscore.1.5.0.dylib`qbs::Internal::RulesApplicator::engine(this=0x000070000029ea90) const + 63 at rulesapplicator.cpp:522
    frame #2: 0x000000010de36bf9 libqbscore.1.5.0.dylib`qbs::Internal::RulesApplicator::runOutputArtifactsScript(this=0x000070000029ea90, inputArtifacts=0x000070000029e830, args=0x000070000029e578) + 713 at rulesapplicator.cpp:372
    frame #3: 0x000000010de34016 libqbscore.1.5.0.dylib`qbs::Internal::RulesApplicator::doApply(this=0x000070000029ea90, inputArtifacts=0x000070000029e830, prepareScriptContext=0x000070000029e8a8) + 2262 at rulesapplicator.cpp:177
    frame #4: 0x000000010de33592 libqbscore.1.5.0.dylib`qbs::Internal::RulesApplicator::applyRule(this=0x000070000029ea90, rule=0x00007fa04accdfa0, inputArtifacts=0x000070000029ebd8) + 1330 at rulesapplicator.cpp:104
    frame #5: 0x000000010de33023 libqbscore.1.5.0.dylib`qbs::Internal::RulesApplicator::applyRuleInEvaluationContext(this=0x000070000029ea90, rule=0x00007fa04accdfa0, inputArtifacts=0x000070000029ebd8) + 131 at rulesapplicator.cpp:79
    frame #6: 0x000000010de31e2f libqbscore.1.5.0.dylib`qbs::Internal::RuleNode::apply(this=0x00007fa04accdf70, logger=0x00007fa04dac2ac0, changedInputs=0x000070000029f348, result=0x000070000029f2c0) + 4015 at rulenode.cpp:127
    frame #7: 0x000000010ddf0660 libqbscore.1.5.0.dylib`qbs::Internal::Executor::executeRuleNode(this=0x00007fa04dac2a80, ruleNode=0x00007fa04accdf70) + 1360 at executor.cpp:458
    frame #8: 0x000000010ddf6416 libqbscore.1.5.0.dylib`qbs::Internal::Executor::visit(this=0x00007fa04dac2a80, ruleNode=0x00007fa04accdf70) + 262 at executor.cpp:987
    frame #9: 0x000000010ddf645f libqbscore.1.5.0.dylib`non-virtual thunk to qbs::Internal::Executor::visit(this=0x00007fa04dac2a80, ruleNode=0x00007fa04accdf70) + 47 at executor.cpp:983
    frame #10: 0x000000010de30d72 libqbscore.1.5.0.dylib`qbs::Internal::RuleNode::accept(this=0x00007fa04accdf70, visitor=0x00007fa04dac2a80) + 34 at rulenode.cpp:59
    frame #11: 0x000000010dded5ce libqbscore.1.5.0.dylib`qbs::Internal::Executor::scheduleJobs(this=0x00007fa04dac2a80) + 1694 at executor.cpp:308
    frame #12: 0x000000010ddeb10b libqbscore.1.5.0.dylib`qbs::Internal::Executor::doBuild(this=0x00007fa04dac2a80) + 2443 at executor.cpp:237
    frame #13: 0x000000010ddea6f8 libqbscore.1.5.0.dylib`qbs::Internal::Executor::build(this=0x00007fa04dac2a80) + 24 at executor.cpp:138
    frame #14: 0x000000010dfb1ecd libqbscore.1.5.0.dylib`qbs::Internal::Executor::qt_static_metacall(_o=0x00007fa04dac2a80, _c=InvokeMetaMethod, _id=3, _a=0x000070000029fe58) + 173 at moc_executor.cpp:105
    frame #15: 0x000000010e83d2e2 QtCore`QMetaObject::activate(QObject*, int, int, void**) + 2994
    frame #16: 0x000000010e661d76 QtCore`QThreadPrivate::start(void*) + 310
    frame #17: 0x00007fff92871c13 libsystem_pthread.dylib`_pthread_body + 131
    frame #18: 0x00007fff92871b90 libsystem_pthread.dylib`_pthread_start + 168
    frame #19: 0x00007fff9286f375 libsystem_pthread.dylib`thread_start + 13