Fix OSGI service unbinding issue

components/device-mgt/io.entgra.device.mgt.core.device.mgt.core/src/main/java/io/entgra/device/mgt/core/device/mgt/core/internal/DeviceManagementDataHolder.java

@@ -118,18 +118,18 @@ public class DeviceManagementDataHolder {
 
     public void setRealmService(RealmService realmService) {
         this.realmService = realmService;
-        this.setTenantManager(realmService);
+        setTenantManager(realmService != null ?
+                realmService.getTenantManager() : null);
     }
 
     public TenantManager getTenantManager() {
-        return tenantManager;
-    }
-
-    private void setTenantManager(RealmService realmService) {
-        if (realmService == null) {
-            throw new IllegalStateException("Realm service is not initialized properly");
+        if (tenantManager == null) {
+            throw new IllegalStateException("Tenant manager is not initialized properly");
         }
-        this.tenantManager = realmService.getTenantManager();
+        return tenantManager;    }
+
+    private void setTenantManager(TenantManager tenantManager) {
+        this.tenantManager = tenantManager;
     }
 
     public DeviceManagementProviderService getDeviceManagementProvider() {