I am trying to list all feature classes from multiple datasets with single GDB. I have tried with below code

```
import arcpy
from arcpy import env
env.workspace = r"C:\GDB\Base.gdb"
fdlist = arcpy.ListDatasets("*","feature")
for fd in fdlist:
    print fd

    env.workspace = fd
    fclist = arcpy.ListFeatureClasses()
    for fc in fclist:
        print(fc)

```

Lists a few feature classes in first dataset then display below error.I have found some scripts that works for single dataset.

Traceback (most recent call last):
File "C:\SOFTWARE\Arcpy\Arcpy\_Script\List\_fc.py", line 47, in 
for fc in fclist:
TypeError: 'NoneType' object is not iterable