WM: check missing space-data & constraints in poll functions

Without this, menu search prints many errors in some contexts.

release/scripts/startup/bl_operators/constraint.py

@@ -33,6 +33,11 @@ class CONSTRAINT_OT_add_target(Operator):
     bl_label = "Add Target"
     bl_options = {'UNDO', 'INTERNAL'}
 
+    @classmethod
+    def poll(cls, context):
+        constraint = getattr(context, "constraint", None)
+        return constraint
+
     def execute(self, context):
         context.constraint.targets.new()
         return {'FINISHED'}
@@ -46,6 +51,11 @@ class CONSTRAINT_OT_remove_target(Operator):
 
     index: IntProperty()
 
+    @classmethod
+    def poll(cls, context):
+        constraint = getattr(context, "constraint", None)
+        return constraint
+
     def execute(self, context):
         tgts = context.constraint.targets
         tgts.remove(tgts[self.index])
@@ -58,6 +68,11 @@ class CONSTRAINT_OT_normalize_target_weights(Operator):
     bl_label = "Normalize Weights"
     bl_options = {'UNDO', 'INTERNAL'}
 
+    @classmethod
+    def poll(cls, context):
+        constraint = getattr(context, "constraint", None)
+        return constraint
+
     def execute(self, context):
         tgts = context.constraint.targets
         total = sum(t.weight for t in tgts)