Only execute file open command during drag-and-drop for dragged files

[slhultgren]

What it does

Fix for DnD files to main area #8739

Solves issue introduced in f840f60

Contributed by STMicroelectronics
Signed-off-by: Samuel HULTGREN samuel.hultgren@st.com

How to test

1, Select text in the "Problems" view, drag into the main area
2, Observe crash in the developer console without the patch:

command.ts:298 Uncaught (in promise) Error: The command 'navigator.open' cannot be executed. There are no active handlers available for the command.
    at CommandRegistry.<anonymous> (command.ts:298)
    at step (command.ts:15)
    at Object.next (command.ts:15)
    at command.ts:15
    at new Promise (<anonymous>)
    at ../../packages/core/lib/common/command.js.__awaiter (command.ts:15)
    at CommandRegistry.../../packages/core/lib/common/command.js.CommandRegistry.executeCommand (command.ts:290)
    at FileNavigatorWidget.<anonymous> (navigator-widget.tsx:110)
    at step (navigator-widget.tsx:15)
    at Object.next (navigator-widget.tsx:15)

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

I confirmed that the pull-request:

• fixes the error when attempting to dnd into the main-area not part of the nav
• the dnd of nav nodes still works correctly (single or multiple files)