@Sami Ahmad
An easy way to return NiFi to a blank canvas, is to simply stop NiFi and remove the flow.xml.gz file from the NiFi's conf directory. When you restart your NiFi a new blank flow.xml.gz file will be generated. Any FlowFiles that had existed in the deleted flow will be purged from NiFi when it is started.
Alternatively:
The error you are seeing is occurring because you are inside a NiFi process group and trying to delete all the components; however, NiFi has detected that there are connections attached to that process group from the process group one level up. NiFi will not allow those components to be removed until there feeding connections are removed first. If you return to the root/top level of your NiFi dataflow you can select the connection entering and existing a process group and delete them. Once they have been deleted, you can select the process group itself and delete it. This will in turn delete all components inside that process group. The deletion of connections will only be allowed if there is no queued FlowFiles in that connection. If there is queued FlowFiles, the FlowFiles must be purged before the connection can be deleted.
Matt
