Ever seen this error? I just came across it today. Thankfully the solution is very simple, but it took a few minutes of Google searches to find the solution.
The problem is caused by having invalid assets selected in your Flex Library Project. Probably you've removed one or more asset files, but the project properties didn't get updated.
To fix this open your project Properties > Flex Library Build Path > Assets, then de-select all your assets, and then re-select the assets. Click OK and it should update your project properties with the correct assets.
Credits go to this useful website: FlexGarden.net