If your Nios® II processor is configured with the "cpu_resetrequest and cpu_resettaken signals" option enabled, you may encounter the above error when you try to simulate the Nios II application.
This is due to the fact that SOPC Builder doesn't drive the signal cpu_resetrequest, and therefore the 'cpu_test_bench/E_valid' signal will be set to 'x' in the generated testbench, which leads to the error.
In order to workaround this issue, modify the testbench to drive the cpu_resetrequest signal in a desired manner.