Remember to set the root sheet before loading

This is done in the frame for GUI loading so we need to explicitly set
this when loading files in the cli

Fixes #17790

(cherry picked from commit f8a25d20)

eeschema/eeschema_helpers.cpp

@@ -126,6 +126,19 @@ SCHEMATIC* EESCHEMA_HELPERS::LoadSchematic( wxString& aFileName, SCH_IO_MGR::SCH
 
     SCHEMATIC* schematic = new SCHEMATIC( project );
 
+    SCH_SHEET* rootSheet = new SCH_SHEET( schematic );
+    schematic->SetRoot( rootSheet );
+
+    SCH_SCREEN* rootScreen = new SCH_SCREEN( schematic );
+    const_cast<KIID&>( rootSheet->m_Uuid ) = rootScreen->GetUuid();
+    schematic->Root().SetScreen( rootScreen );
+
+
+    schematic->RootScreen()->SetFileName( wxEmptyString );
+
+    // Don't leave root page number empty
+    schematic->RootScreen()->SetPageNumber( wxT( "1" ) );
+
     wxFileName schFile = aFileName;
     schFile.MakeAbsolute();