If anyone is still experiencing a crash with the nwjs crash after resizing the preview window, closing then opening it again (or just experiencing the nwjs crash when in the middle of using the dev tools), feel free to do this until Scirra update their nwjs version for Construct 3 Desktop (I know this issue has been very disruptive for me):
First of all as a precautionary measure. Save a back up of your project as a separate file (or folder) and continue using that until scirra update the nwjs.
Second of all: copy the folder that you have of Construct 3 as a backup in case something happens with your installation (worse case scenario, you can always just download the old version from here again anyway. I don't think you'll run into any problems).
Third: download this: dl.nwjs.io/v0.40.1/nwjs-sdk-v0.40.1-win-x64.zip
Fourth: Extract all of the files EXCEPT FOR THE .EXE FILE (keep the Construct3.exe unchanged. If you change it to the new one, it will not load any saved projects so it's important this file is unchanged).
Fifth: Run Construct 3. You will be able to use your dev tools (press F12), close and open the preview window as much as you want without it crashing the nwjs.
I hope this is is helpful.
Thanks to Forbidden_Soul from Twitch for helping me with this issue. I can finally test my game without experiencing a crash every 30 seconds.