API: check if there's a circuit for Circuit_Save.

Author: PMeira

docs/changelog.md

@@ -22,6 +22,7 @@
 - Alt/CE: Fix `RegisterValues` in the C header.
 - Alt/PDE: Check for missing solution in a few functions.
 - Commands/`CalcLaplacian`: Give proper error message instead of crashing or giving "access violation" messages.
+- `Circuit_Save`: check if there is circuit to save.
 
 ## Version 0.14.1 (2024-02-16)
 

src/CAPI/CAPI_Circuit.pas

@@ -1143,6 +1143,8 @@ function Circuit_Save(dirfilepath: PAnsiChar; saveFlags: DSSSaveFlags): PAnsiCha
 begin
     res := '';
     Result := NIL;
+    if InvalidCircuit(DSSPrime) then
+        Exit;
     if DSSPrime.ActiveCircuit.Save(dirfilepath, @saveFlags, @res) then
         Result := DSS_GetAsPAnsiChar(DSSPrime, res);
 end;