tkpanels: work around tkinter bug cancelling file open dialog on macOS

Fixes #811

direct/src/tkpanels/AnimPanel.py

@@ -276,7 +276,7 @@ class AnimPanel(AppShell):
             title = 'Load Animation',
             parent = self.component('hull')
             )
-        if not animFilename:
+        if not animFilename and animFilename != 'None':
             # no file selected, canceled
             return
 

direct/src/tkpanels/MopathRecorder.py

@@ -1638,7 +1638,7 @@ class MopathRecorder(AppShell, DirectObject):
             initialdir = path,
             title = 'Load Nurbs Curve',
             parent = self.parent)
-        if mopathFilename:
+        if mopathFilename and mopathFilename != 'None':
             self.reset()
             nodePath = loader.loadModel(
                 Filename.fromOsSpecific(mopathFilename))

direct/src/tkpanels/ParticlePanel.py

@@ -1272,7 +1272,7 @@ class ParticlePanel(AppShell):
             initialdir = path,
             title = 'Load Particle Effect',
             parent = self.parent)
-        if particleFilename:
+        if particleFilename and particleFilename != 'None':
             # Delete existing particles and forces
             self.particleEffect.loadConfig(
                 Filename.fromOsSpecific(particleFilename))