Build error referencing build.xml and proguard file: "null returned: 1"

Verify your JAVA_HOME variable is correctly set to some path like C:\Program Files\Java\jdk1.7.0_67\ and if the error still goes on, it may be solved going to the build.xml and setting true to the variable fork, or the passed variable ${need.javac.fork}, which sets fork´s value in:

fork="${need.javac.fork}"

For me: I had

edited my config.xml wrongly

. Try undoing your edits or placing a generic config.xml then see if she builds.

Close Eclipse and then run ant clean in your project folder.