I have found that this happens after clicking on the "Create Actions..." button, and saving a new inputactions file. The workaround is to simply click on the target button next to the Actions field on the component, and set it to the new inputactions file that you just created. You can then continue setting additional properties in the component as required.

I might have a solution/work around for this error. I'm running 2020.2.2f1 with Input System 1.0.2. After creating the actions by clicking "Create Actions..." inside of a player input component, we are going to manually make a copy of it (copy/paste or duplicating does not work). To do this just navigate to the project tab and in whatever folder right click, create, and then input actions. This method creates a completely blank input actions file. Double click the file and it will open the input actions window.
In this window you can set up the inputs exactly like the inputs you made when you clicked the "Create Actions..." button inside the player input component.
To do this, open the input action window for the first input actions you created and use it as a reference. In the second input actions you created you need to add control schemes by selecting the drop down menu in the top left corner. Make sure the control schemes that you make are the same as the ones from the first input actions. After your input schemes have been added just copy and paste the actions from the first one over to the new one. Close the first input actions window and delete its file. Now save the second input actions and it will not generate any errors. I hope I explained this clearly so that it was easy to follow and helped anybody that had this error.

I hope this is fixed soon, Input System is awesome but this issue renders it completely unusable from the Editor even in 2019.4 LTS. It still works if you only interact with it in code, but for my simpler projects that makes it more trouble than it's worth.

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