Export to QuickStatements sometimes produces an empty window

Describe the bug
Sometimes, when I hit "Export to QuickStatements" it just throws an empty window, without any text to copy to QuickStatements. If I go back, look again at the schema (without editing it), change views a couple of times, then try to export again, this time it seems to work. (Sorry, can't be more precise, but it's happened several times to me today. Possibly related to #1700 - on at least some of the occasions it is possible I had re-started OpenRefine & gone to an existing project with an existing schema.

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Current Results
What results occured or were shown.

Expected behavior
A clear and concise description of what you expected to happen or to show.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: Windows 10, release 1809
• Browser Version: Chrome 79.0.3945.117
• JRE or JDK Version: JRE 1.8.0_231

OpenRefine (please complete the following information):

• Version : OpenRefine 3.3 rc 1

Datasets
If you are allowed and are OK with making your data public, it would be awesome if you can include or attach the data causing the issue or a URL pointing to where the data is.
If you are concerned about keeping your data private, ping us on our mailing list

Additional context
Add any other context about the problem here.

Start from the “Export to QuickStatements” action and reproduce the failure with an existing project and schema, including the reported restart and view-change sequence. Trace the export entry point and reduce the intermittent behavior to a repeatable case; done means the export window consistently contains the expected QuickStatements text.