fix(nrtk_transforms): avoid runtime error from type hints

src/nrtk_explorer/library/nrtk_transforms.py

@@ -1,4 +1,4 @@
-from typing import Any, Dict, Union
+from typing import Any, Dict, Optional, TYPE_CHECKING
 
 import numpy as np
 import logging
@@ -15,22 +15,27 @@
     from pybsm.otf import darkCurrentFromDensity
     from nrtk.impls.perturb_image.generic.cv2.blur import GaussianBlurPerturber
     from nrtk.impls.perturb_image.pybsm.perturber import PybsmPerturber, PybsmSensor, PybsmScenario
-
-    GaussianBlurPerturberType = Union[GaussianBlurPerturber, None]
-    PybsmPerturberType = Union[PybsmPerturber, None]
 except ImportError:
     logger.info("Disabling NRTK transforms due to missing library/failing imports")
     ENABLED_NRTK_TRANSFORMS = False
+
+if TYPE_CHECKING:
+    GaussianBlurPerturberType = GaussianBlurPerturber
+    PybsmPerturberType = PybsmPerturber
+else:
     GaussianBlurPerturberType = None
     PybsmPerturberType = None
 
+GaussianBlurPerturberArg = Optional[GaussianBlurPerturberType]
+PybsmPerturberArg = Optional[PybsmPerturberType]
+
 
 def nrtk_transforms_available():
     return ENABLED_NRTK_TRANSFORMS
 
 
 class NrtkGaussianBlurTransform(ImageTransform):
-    def __init__(self, perturber: GaussianBlurPerturberType = None):
+    def __init__(self, perturber: GaussianBlurPerturberArg = None):
         if perturber is None:
             perturber = GaussianBlurPerturber()
 
@@ -160,7 +165,7 @@ def createSampleSensorAndScenario():
 
 
 class NrtkPybsmTransform(ImageTransform):
-    def __init__(self, perturber: PybsmPerturberType = None):
+    def __init__(self, perturber: PybsmPerturberArg = None):
         if perturber is None:
             sensor, scenario = createSampleSensorAndScenario()
             perturber = PybsmPerturber(sensor=sensor, scenario=scenario)