Avoid crashes when, for some reason, a dll cannot be loaded.

In this case a null pointer was returned by the internal code.
This pointer is now tested against nullptr to avoid the application crashing.
Fixes #12080
https://gitlab.com/kicad/code/kicad/issues/12080

eeschema/sch_edit_frame.cpp

@@ -945,6 +945,12 @@ void SCH_EDIT_FRAME::OnUpdatePCB( wxCommandEvent& event )
         fn.SetExt( PcbFileExtension );
 
         frame = Kiway().Player( FRAME_PCB_EDITOR, true );
+
+        // If Kiway() cannot create the Pcbnew frame, it shows a error message, and
+        // frame is null
+        if( !frame )
+            return;
+
         frame->OpenProjectFiles( std::vector<wxString>( 1, fn.GetFullPath() ) );
     }
 
@@ -1137,6 +1143,12 @@ void SCH_EDIT_FRAME::OnOpenPcbnew( wxCommandEvent& event )
             if( !frame )
             {
                 frame = Kiway().Player( FRAME_PCB_EDITOR, true );
+
+                // frame can be null if Cvpcb cannot be run. No need to show a warning
+                // Kiway() generates the error messages
+                if( !frame )
+                    return;
+
                 frame->OpenProjectFiles( std::vector<wxString>( 1, boardfn.GetFullPath() ) );
             }
 
@@ -1174,6 +1186,12 @@ void SCH_EDIT_FRAME::OnOpenCvpcb( wxCommandEvent& event )
         if( !player )
         {
             player = Kiway().Player( FRAME_CVPCB, true );
+
+            // player can be null if Cvpcb cannot be run. No need to show a warning
+            // Kiway() generates the error messages
+            if( !player )
+                return;
+
             player->Show( true );
         }
 

pcbnew/pcb_edit_frame.cpp

@@ -1549,15 +1549,20 @@ void PCB_EDIT_FRAME::ToPlotter( int aID )
 }
 
 
-bool PCB_EDIT_FRAME::TestStandalone()
+int PCB_EDIT_FRAME::TestStandalone()
 {
     if( Kiface().IsSingle() )
-        return false;
+        return 0;
 
     // Update PCB requires a netlist. Therefore the schematic editor must be running
     // If this is not the case, open the schematic editor
     KIWAY_PLAYER* frame = Kiway().Player( FRAME_SCH, true );
 
+    // If Kiway() cannot create the eeschema frame, it shows a error message, and
+    // frame is null
+    if( !frame )
+        return -1;
+
     if( !frame->IsShown() )
     {
         wxFileName fn( Prj().GetProjectPath(), Prj().GetProjectName(),
@@ -1572,7 +1577,7 @@ bool PCB_EDIT_FRAME::TestStandalone()
             if( !fn.FileExists() )
             {
                 DisplayError( this, _( "The schematic for this board cannot be found." ) );
-                return false;
+                return -2;
             }
         }
 
@@ -1586,14 +1591,14 @@ bool PCB_EDIT_FRAME::TestStandalone()
         Raise();
     }
 
-    return true;            //Success!
+    return 1;            //Success!
 }
 
 
 bool PCB_EDIT_FRAME::FetchNetlistFromSchematic( NETLIST& aNetlist,
                                                 const wxString& aAnnotateMessage )
 {
-    if( !TestStandalone() )
+    if( TestStandalone() == 0 )
     {
         DisplayErrorMessage( this, _( "Cannot update the PCB because PCB editor is opened in "
                                       "stand-alone mode. In order to create or update PCBs from "
@@ -1602,6 +1607,9 @@ bool PCB_EDIT_FRAME::FetchNetlistFromSchematic( NETLIST& aNetlist,
         return false;       // Not in standalone mode
     }
 
+    if( TestStandalone() < 0 )      // Problem with Eeschema or the schematic
+        return false;
+
     Raise();                // Show
 
     std::string payload( aAnnotateMessage );
@@ -1691,6 +1699,11 @@ void PCB_EDIT_FRAME::RunEeschema()
             }
         }
 
+        // If Kiway() cannot create the eeschema frame, it shows a error message, and
+        // frame is null
+        if( !frame )
+            return;
+
         if( !frame->IsShown() ) // the frame exists, (created by the dialog field editor)
                                 // but no project loaded.
         {

pcbnew/pcb_edit_frame.h

@@ -611,9 +611,11 @@ public:
     /**
      * Test if standalone mode.
      *
-     * @return true if in standalone, opens Eeschema, and opens the schematic for this project
+     * @return 0 if in standalone, -1 if Eeschema cannot be opened,
+     * -2 if the schematic cannot be opened and 1 if OK.
+     * If OK, opens Eeschema, and opens the schematic for this project
      */
-    bool TestStandalone( void );
+    int TestStandalone( void );
 
     /**
      * Read a netlist from a file into a #NETLIST object.