Theia: `All Files` filter option throws error

[colin-grant-work]

Bug Description:

When a file dialog is open that includes a filter field, selecting All Files will throw an error in the console.

Steps to Reproduce:

1. Start Theia on master in a browser.
2. Select File menu -> Open Workspace
3. At the bottom of the file picker, select one of the filter options
4. Then select the All Files filter option
5. Observe that an error about .slice of undefined is thrown.

I'll create a PR to fix this in a moment.

Additional Information

• Operating System: REL7
• Theia Version: Current master

[vince-fugnitto]

I was able to reproduce :smiley:

react-dom.development.js:327 Uncaught TypeError: Cannot read property 'slice' of undefined
    at FileDialogTree.push.../../packages/filesystem/lib/browser/file-dialog/file-dialog-tree.js.FileDialogTree.setFilter (file-dialog-tree.ts:36)
    at FileDialogTreeFiltersRenderer.push.../../packages/filesystem/lib/browser/file-dialog/file-dialog-tree-filters-renderer.js.FileDialogTreeFiltersRenderer.onFilterChanged (file-dialog-tree-filters-renderer.tsx:71)
    at FileDialogTreeFiltersRenderer._this.handleFilterChanged (file-dialog-tree-filters-renderer.tsx:46)
    at HTMLUnknownElement.callCallback (react-dom.development.js:188)
    at Object.invokeGuardedCallbackDev (react-dom.development.js:237)
    at invokeGuardedCallback (react-dom.development.js:292)
    at invokeGuardedCallbackAndCatchFirstError (react-dom.development.js:306)
    at executeDispatch (react-dom.development.js:389)
    at executeDispatchesInOrder (react-dom.development.js:414)
    at executeDispatchesAndRelease (react-dom.development.js:3278)

[colin-grant-work]

Unusual to get a smiley face for an error 🐛 😸