Photo crash on project switch

[PeterPetrik]

fix #2879

When testing on test simple QML project and our implementation StackView, it looks like
when stackView.clear() is called:

• StackView.onRemoved is called first
• Some signals are passed and evaluated (e.g. QGIS project is loaded, layer in featurelayerpair contains dangling pointer and in evaluateExpression it crashes)
• Only after then Component.onDestruction is called

Therefore we need to remove all connections to layer/project/etc. we are about to unload from all existing forms (there should be only one in the stack) before calling clear()

Other chances in the C++ code are probably because we are passing FeatureLayerPair with nullptr layer to the attribute controller, so we should make sure it does not crash by dereferencing null layer pointer.

Sample QML code

import QtQuick
import QtQuick.Controls
import QtQuick.Window

Window {
  width: 640
  height: 480
  visible: true
  title: qsTr("Hello World")

  Row {
    anchors.fill: parent
    spacing: 2

  Button {
    text: "add"
    onClicked: stackView.push(formComponent)
  }

  Button {
    text: "clean"
    onClicked: stackView.clear()
  }

  StackView {

    height: 100
    width: 100

    id: stackView

    focus: true

    // anchors.fill: parent
  }

  Component {
    id: formComponent

    Rectangle {
      id: wrapper

      height:100
      width: 100
      color: "red"

      StackView.onRemoved: {
        console.log("*****StackView.onRemoved")
      }

      Component.onCompleted: {
        console.log("*****Created form")
      }

      Component.onDestruction: {
        console.log("*****Destroying form")
      }
    }
  }

  }


}

[PeterPetrik]

@jozef-budac testing instructions in the issue ticket #2879

[jozefbudac]

Verified on builded APK on Android OS