qmllint required semicolon when changing variable declaration to `let`

I changed a line of code that defines a regular expression to declare a variable with `let` instead of `var`, and qmllint caused the build to fail. After changing it back to `var`, the build succeeded. In both cases, the code ran fine and didn't throw any errors when I tested it.

Using a linux docker in Bitbucket Pipelines, the following was run to install qmllint and test the codebase:

apt-get install -y qt5-default qtdeclarative5-dev-tools
find -type f -name \*qml -exec qmllint \{\} +

Here was the output:

./qml/GControls/GSpinBox11.qml:68 : Expected token `;'

Here's the code that caused the problem (used inside a SpinBox):

validator: RegExpValidator {
    regExp: {
        let expression = "\\d+"