GerbView: Fix selection of the last two grid settings

Fixes: lp:1827642
* https://bugs.launchpad.net/kicad/+bug/1827642

(cherry picked from commit ba19d93df9)

common/legacy_wx/eda_draw_frame.cpp

@@ -469,6 +469,9 @@ void EDA_DRAW_FRAME::OnSelectGrid( wxCommandEvent& event )
         int index = m_gridSelectBox->GetSelection();
         wxASSERT( index != wxNOT_FOUND );
 
+        // GerbView does not support custom grid
+        if( m_Ident != FRAME_GERBER )
+        {
             if( index == int( m_gridSelectBox->GetCount() - 2 ) )
             {
                 // this is the separator
@@ -484,6 +487,7 @@ void EDA_DRAW_FRAME::OnSelectGrid( wxCommandEvent& event )
                 OnGridSettings( dummy2 );
                 return;
             }
+        }
 
         clientData = (int*) m_gridSelectBox->wxItemContainer::GetClientData( index );
 
@@ -668,7 +672,13 @@ void EDA_DRAW_FRAME::SetPresetGrid( int aIndex )
 
     if( m_gridSelectBox )
     {
-        if( glistIdx < 0 || glistIdx >= (int) m_gridSelectBox->GetCount() - 2 )
+        int highestGrid = ( int )m_gridSelectBox->GetCount();
+
+        // GerbView does not support the user grid setting
+        if( m_Ident != FRAME_GERBER )
+            highestGrid -= 2;
+
+        if( glistIdx < 0 || glistIdx >= highestGrid )
         {
             wxASSERT_MSG( false, "Invalid grid index" );
             return;