[Bug] When using a custom map style, HTML/JSON exports are missing imported data, created layers and custom map styles

**Describe the bug**
If a custom map style is used as the base map, exporting as HTML and JSON is broken.
When importing an exported kepler JSON file or opening the HTML file in the browser, the default kepler.gl editor is displayed and any of the data, layers and custom map styles from the original map are not present.

This bug does not occur when the default base map is used.

**To Reproduce**
Steps to reproduce the behavior:
1. Upload csv data
2. Style the map layer for the data
3. Import a custom map style and paste mapbox key
4. Export map as HTML or JSON and paste mapbox key
5. Open HTML file in browser

**Expected behavior**
I expect the export feature to include any uploaded data, map layers and custom map styles in the map export

**Desktop (please complete the following information):**
- OS: macOS
- Browser: Chrome and Firefox
- Version 89/88

Start by reproducing the issue with uploaded CSV data, a styled layer, and a custom map style, then inspect the HTML and JSON export paths. Compare exports using the default and custom base maps. Done means importing the JSON or opening the HTML preserves the uploaded data, layers, and custom map style instead of showing the default editor.

Written by the indexing model from the issue text.