check for invalid ValidatorManager in initialize

contracts/validator-manager/PoAValidatorManager.sol

@@ -64,6 +64,9 @@ contract PoAValidatorManager is IPoAValidatorManager, OwnableUpgradeable {
         internal
         onlyInitializing
     {
+        // Ensure the manager is valid by calling it
+        manager.getChurnPeriodSeconds();
+
         PoAValidatorManagerStorage storage $ = _getPoAValidatorManagerStorage();
         $._manager = manager;
     }

contracts/validator-manager/tests/ERC20TokenStakingManagerTests.t.sol

@@ -161,6 +161,14 @@ contract ERC20TokenStakingManagerTest is PoSValidatorManagerTest {
         app.initialize(defaultPoSSettings, token);
     }
 
+    function testInvalidValidatorManager() public {
+        app = new ERC20TokenStakingManager(ICMInitializable.Allowed);
+        vm.expectRevert();
+
+        // PoSValidatorManagerSettings.manager is not set
+        app.initialize(_defaultPoSSettings(), token);
+    }
+
     function testInvalidValidatorMinStakeDuration() public {
         uint256 stakeAmount = _weightToValue(DEFAULT_WEIGHT);
         vm.expectRevert(

contracts/validator-manager/tests/NativeTokenStakingManagerTests.t.sol

@@ -144,6 +144,14 @@ contract NativeTokenStakingManagerTest is PoSValidatorManagerTest {
         app.initialize(defaultPoSSettings);
     }
 
+    function testInvalidValidatorManager() public {
+        app = new NativeTokenStakingManager(ICMInitializable.Allowed);
+        vm.expectRevert();
+
+        // PoSValidatorManagerSettings.manager is not set
+        app.initialize(_defaultPoSSettings());
+    }
+
     // Helpers
     function _initiateValidatorRegistration(
         bytes memory nodeID,

contracts/validator-manager/tests/PoAValidatorManagerTests.t.sol

@@ -39,6 +39,13 @@ contract PoAValidatorManagerTest is ValidatorManagerTest {
         app.initialize(validatorManager, address(this));
     }
 
+    function testInvalidValidatorManager() public {
+        app = new PoAValidatorManager(ICMInitializable.Allowed);
+        vm.expectRevert();
+
+        app.initialize(ValidatorManager(address(0)), address(this));
+    }
+
     function testInvalidOwnerRegistration() public {
         vm.prank(vm.addr(1));
         vm.expectRevert(