[REG 5.12.2->5.12.3] QML: The loss of "this" context in a function

In some cases, "this" context of the directly called function is incorrect.

Error in an example : Cannot assign QJSValue to QQuickItem*

How to repeat:

• Launch attached example test_bug.zip
• Wait for display text and green rectangle 
• Click on any area of a window
• EXPECTED: get not empty this context
• ACTUAL: in log error: Cannot assign QJSValue to QQuickItem*

Regression commit in qtdeclarative module, link https://code.qt.io/cgit/qt/qtdeclarative.git/commit/?id=bcbce96fffd25a4f2810f03cec060ab13e34ac9e