When using a QObject in one QML engine, destructing the engine, and using the same object in another QML engine, the second access to the QObject causes a TypeError of undefined object.
Attached is a simple test program that can be used to reproduce the issue. When running the program with QV4_MM_AGGRESSIVE_GC env variable set, the following error gets printed when accessing the testObject second time from the second QML engine:
test.qml:8: TypeError: Cannot read property 'objectName' of undefined
The issue can be reproduced with the current HEAD from 5.5 branch (commit 31ee12e). Haven't tried with 5.5.0 beta but I guess this can be reproduced also there.