Added null checks for device cache in-case cache is disabled.

components/device-mgt/org.wso2.carbon.device.mgt.core/src/main/java/org/wso2/carbon/device/mgt/core/cache/impl/DeviceCacheManagerImpl.java

@@ -55,6 +55,7 @@ public class DeviceCacheManagerImpl implements DeviceCacheManager {
     @Override
     public void addDeviceToCache(DeviceIdentifier deviceIdentifier, Device device, int tenantId) {
         Cache<DeviceCacheKey, Device> lCache = DeviceManagerUtil.getDeviceCache();
+        if (lCache != null) {
             DeviceCacheKey cacheKey = getCacheKey(deviceIdentifier, tenantId);
             if (lCache.containsKey(cacheKey)) {
                 this.updateDeviceInCache(deviceIdentifier, device, tenantId);
@@ -62,40 +63,50 @@ public class DeviceCacheManagerImpl implements DeviceCacheManager {
                 lCache.put(cacheKey, device);
             }
         }
+    }
 
     @Override
     public void removeDeviceFromCache(DeviceIdentifier deviceIdentifier, int tenantId) {
         Cache<DeviceCacheKey, Device> lCache = DeviceManagerUtil.getDeviceCache();
+        if (lCache != null) {
             DeviceCacheKey cacheKey = getCacheKey(deviceIdentifier, tenantId);
             if (lCache.containsKey(cacheKey)) {
                 lCache.remove(cacheKey);
             }
         }
+    }
 
     @Override
     public void removeDevicesFromCache(List<DeviceCacheKey> deviceList) {
         Cache<DeviceCacheKey, Device> lCache = DeviceManagerUtil.getDeviceCache();
+        if (lCache != null) {
             for (DeviceCacheKey cacheKey : deviceList) {
                 if (lCache.containsKey(cacheKey)) {
                     lCache.remove(cacheKey);
                 }
             }
         }
+    }
 
     @Override
     public void updateDeviceInCache(DeviceIdentifier deviceIdentifier, Device device, int tenantId) {
         Cache<DeviceCacheKey, Device> lCache = DeviceManagerUtil.getDeviceCache();
+        if (lCache != null) {
             DeviceCacheKey cacheKey = getCacheKey(deviceIdentifier, tenantId);
             if (lCache.containsKey(cacheKey)) {
                 lCache.replace(cacheKey, device);
             }
         }
+    }
 
     @Override
     public Device getDeviceFromCache(DeviceIdentifier deviceIdentifier, int tenantId) {
         Cache<DeviceCacheKey, Device> lCache = DeviceManagerUtil.getDeviceCache();
+        if (lCache != null) {
             return lCache.get(getCacheKey(deviceIdentifier, tenantId));
         }
+        return null;
+    }
 
 
     private DeviceCacheKey getCacheKey(DeviceIdentifier deviceIdentifier, int tenantId) {