editor: 'save as' does not work correctly

[vince-fugnitto]

Bug Description:

There is an issue with the 'save as functionality where:

• performing save as on an editor does not close the original editor
• performing save for untitled editors (ultimately a save as) the original untitled editor is preserved

untitled-error.mp4

root WARN TypeError: Cannot read properties of undefined (reading 'create')
    at createTextBuffer (file:///home/evinfug/workspaces/theia/examples/electron/lib/vendors-node_modules_theia_monaco-editor-core_esm_vs_editor_editor_main_js.js:93904:20)
    at TextModel.setValue (file:///home/evinfug/workspaces/theia/examples/electron/lib/vendors-node_modules_theia_monaco-editor-core_esm_vs_editor_editor_main_js.js:94130:44)
    at MonacoEditorModel.applySnapshot (file:///home/evinfug/workspaces/theia/examples/electron/lib/node_modules_theia_monaco-editor-core_esm_vs_base_common_sync_recursive-packages_monaco_lib_b-6d7964.js:565:20)
    at FilesystemSaveResourceService.copyAndSave (file:///home/evinfug/workspaces/theia/examples/electron/lib/packages_filesystem_lib_browser_breadcrumbs_filepath-breadcrumb_js-packages_filesystem_lib_br-eff824.js:842:28)
    at async FilesystemSaveResourceService.saveAs (file:///home/evinfug/workspaces/theia/examples/electron/lib/packages_filesystem_lib_browser_breadcrumbs_filepath-breadcrumb_js-packages_filesystem_lib_br-eff824.js:816:17)
    at async FilesystemSaveResourceService.save (file:///home/evinfug/workspaces/the

Steps to Reproduce:

1. start the application
2. execute the command new file - an untitled editor should open
3. add content to the untitled editor, and save - notice the editor is kept open and dirty with a new editor opened
4. execute the command save as on a regular editor
5. after performing the save as the original editor is not replaced with the new one