Fix crash when realizing instance of OpenGL

The FOOTPRINT_PREVIEW_PANEL may instantiate prior to any other checks,
we need to ensure that this widget is realized on Linux prior to getting
the OpenGL calls otherwise GTKGetDrawingWindow may return NULL
incorrectly

common/gal/opengl/opengl_gal.cpp

@@ -43,6 +43,7 @@
 #include <pgm_base.h>
 #include <trace_helpers.h>
 
+#include <wx/app.h>
 #include <wx/frame.h>
 
 #include <macros.h>
@@ -498,6 +499,13 @@ wxString OPENGL_GAL::CheckFeatures( GAL_DISPLAY_OPTIONS& aOptions )
         testFrame->Raise();
         testFrame->Show();
 
+#ifdef __WXGTK__
+        // On GTK, Show() only queues realization. The GDK drawing window
+        // needed by SetCurrent() may not exist yet. Yield to let the event
+        // loop process the realize signal before we try to lock the context.
+        wxYield();
+#endif
+
         GAL_CONTEXT_LOCKER lock( opengl_gal );
         opengl_gal->init();
     }
@@ -2837,8 +2845,14 @@ void OPENGL_GAL::init()
     if( glVersion == 0 )
         throw std::runtime_error( "Failed to load OpenGL via loader" );
 
-    SetOpenGLInfo( (const char*) glGetString( GL_VENDOR ), (const char*) glGetString( GL_RENDERER ),
-                   (const char*) glGetString( GL_VERSION ) );
+    const char* vendor = (const char*) glGetString( GL_VENDOR );
+    const char* renderer = (const char*) glGetString( GL_RENDERER );
+    const char* version = (const char*) glGetString( GL_VERSION );
+
+    if( !version )
+        throw std::runtime_error( "No GL context is current (glGetString returned NULL)" );
+
+    SetOpenGLInfo( vendor, renderer, version );
 
     // Check the OpenGL version (minimum 2.1 is required)
     if( !GLAD_GL_VERSION_2_1 )