Try deleting the build folder. Make sure you make a copy of the project first. The build folder is used by Grade to build the project source and its dependencies in to a project. Deleting this directory is generally safe, and sometimes as you have experienced, this is necessary as it can get in a problematic state.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…