Crash is persistent and can occur both on file open and during active code changes. 

To reproduce the crash, you can use the following code snippet:

        Rectangle {
            id: rectangle2
            width: 200
            height: 200
            color: "#3485ca"
            Connections {
                target: rectangle2
                onVisibleChanged: {
                    if (rectangle2.visible) {
                        console.log("visible")
                    } else {
                        console.log("invisible")
                    }
                    console.log("crash") //because of this line
                }
            }
       }

Or you can easily cause it yourself by adding code (like console.log()) to any Connection with a conditional statement.

This crash occurs even if you are not using Connections Editor, just during code editing or file opening. 
This is 100% valid QML, and a common debugging pattern, so it might break QDS usage for existing projects.

Else statement is not important, only having something after if(...) { ... } block is.
Having anything before if statement seems to be fine and doesn't seem to be causing any crashes.