Python scripting: fix a crash with some pythons scripts calling BOARD::ComputeBoundingBox()

This function calls Pgm(), but when running from a script (not from kicad) Pgm() uses a nullptr reference.
The nullptr reference is now tested in ComputeBoundingBox()

cvpcb/cvpcb.cpp

@@ -105,6 +105,14 @@ PGM_BASE& Pgm()
 }
 
 
+// Similar to PGM_BASE& Pgm(), but return nullptr when a *.ki_face
+// is run from a python script, mot from a Kicad application
+PGM_BASE* PgmOrNull()
+{
+    return process;
+}
+
+
 //!!!!!!!!!!!!!!! This code is obsolete because of the merge into pcbnew, don't bother with it.
 
 FP_LIB_TABLE GFootprintTable;

include/pgm_base.h

@@ -424,4 +424,9 @@ protected:
 /// Implemented in: 1) common/single_top.cpp,  2) kicad/kicad.cpp, and 3) scripting/kiway.i
 extern PGM_BASE& Pgm();
 
+/// similat to PGM_BASE& Pgm(), but return a reference that can be nullptr
+/// when running a shared lib from a script, not from a kicad appl
+extern PGM_BASE* PgmOrNull();
+
+
 #endif  // PGM_BASE_H_

pcbnew/class_board.cpp

@@ -761,7 +761,8 @@ EDA_RECT BOARD::ComputeBoundingBox( bool aBoardEdgesOnly ) const
 {
     EDA_RECT area;
     LSET     visible = GetVisibleLayers();
-    bool     showInvisibleText = IsElementVisible( LAYER_MOD_TEXT_INVISIBLE ) && !Pgm().m_Printing;
+    bool     showInvisibleText = IsElementVisible( LAYER_MOD_TEXT_INVISIBLE )
+                                 && PgmOrNull() && !PgmOrNull()->m_Printing;
 
     // Check segments, dimensions, texts, and fiducials
     for( auto item : m_drawings )

pcbnew/pcbnew.cpp

@@ -192,6 +192,14 @@ PGM_BASE& Pgm()
     wxASSERT( process );    // KIFACE_GETTER has already been called.
     return *process;
 }
+
+// Similar to PGM_BASE& Pgm(), but return nullptr when a *.ki_face
+// is run from a python script, mot from a Kicad application
+PGM_BASE* PgmOrNull()
+{
+    return process;
+}
+
 #endif