Don't try to resolve variables before the schematic is set up.

Fixes https://gitlab.com/kicad/code/kicad/issues/8746

eeschema/sch_sheet.cpp

@@ -695,7 +695,7 @@ int SCH_SHEET::CountSheets() const
 
     if( m_screen )
     {
-        for( auto aItem : m_screen->Items().OfType( SCH_SHEET_T ) )
+        for( SCH_ITEM* aItem : m_screen->Items().OfType( SCH_SHEET_T ) )
             count += static_cast<SCH_SHEET*>( aItem )->CountSheets();
     }
 

eeschema/schematic.cpp

@@ -122,6 +122,8 @@ SCH_SCREEN* SCHEMATIC::RootScreen() const
 
 bool SCHEMATIC::ResolveTextVar( wxString* token, int aDepth ) const
 {
+    if( !CurrentSheet().empty() )
+    {
         if( token->IsSameAs( wxT( "#" ) ) )
         {
             *token = CurrentSheet().GetPageNumber();
@@ -145,6 +147,9 @@ bool SCHEMATIC::ResolveTextVar( wxString* token, int aDepth ) const
         }
 
         return CurrentSheet().LastScreen()->GetTitleBlock().TextVarResolver( token, m_project );
+    }
+
+    return false;
 }