Cater for at-genesis best headers (#3264)

packages/api-derive/src/tx/signingInfo.ts

@@ -35,13 +35,16 @@ function signingHeader (api: ApiInterfaceRx): Observable<Header> {
     api.rpc.chain.getHeader(),
     api.rpc.chain.getFinalizedHead()
   ]).pipe(
-    switchMap(([{ parentHash }, finHash]) =>
+    switchMap(([bestHeader, finHash]) =>
       // retrieve the headers - in the case of the current block, we use the parent
       // to minimize (not completely remove) the impact that forks do have on the system
-      combineLatest([
-        api.rpc.chain.getHeader(parentHash),
-        api.rpc.chain.getHeader(finHash)
-      ])
+      // (when at genesis, just return the current header as the last known)
+      bestHeader.parentHash.isEmpty
+        ? of([bestHeader, bestHeader])
+        : combineLatest([
+          api.rpc.chain.getHeader(bestHeader.parentHash),
+          api.rpc.chain.getHeader(finHash)
+        ])
     ),
     map(([current, finalized]) =>
       // determine the hash to use, current when lag > max, else finalized