design blocks: don't crash on missing preview widget

Copied from footprint preview widget, which has the same issue

common/widgets/design_block_preview_widget.h

@@ -45,6 +45,11 @@ public:
     DESIGN_BLOCK_PREVIEW_WIDGET( wxWindow* aParent ) : wxPanel( aParent ) {}
     ~DESIGN_BLOCK_PREVIEW_WIDGET() = default;
 
+    /**
+     * Return whether the widget initialized properly. This could return false if Kiway is
+     * not available. If this returns false, no other methods should be called.
+     */
+    virtual bool IsInitialized() const = 0;
 
     /**
      * Set the contents of the status label and display it.

common/widgets/panel_design_block_chooser.cpp

@@ -318,6 +318,9 @@ void PANEL_DESIGN_BLOCK_CHOOSER::onOpenLibsTimer( wxTimerEvent& aEvent )
 
 void PANEL_DESIGN_BLOCK_CHOOSER::onDesignBlockSelected( wxCommandEvent& aEvent )
 {
+    if( !m_preview || !m_preview->IsInitialized() )
+        return;
+
     if( GetSelectedLibId().IsValid() )
     {
         std::unique_ptr<DESIGN_BLOCK> designBlock( m_parent->GetDesignBlock( GetSelectedLibId(), true, true ) );

eeschema/widgets/sch_design_block_preview_widget.h

@@ -47,6 +47,12 @@ public:
 
     ~SCH_DESIGN_BLOCK_PREVIEW_WIDGET() override;
 
+    /*
+     * Return whether the widget initialized properly. This could return false if Kiway is
+     * not available. If this returns false, no other methods should be called.
+     */
+    bool IsInitialized() const override { return m_preview != nullptr; }
+
     /**
      * Set the contents of the status label and display it.
      */

pcbnew/widgets/pcb_design_block_preview_widget.h

@@ -47,6 +47,12 @@ public:
 
     ~PCB_DESIGN_BLOCK_PREVIEW_WIDGET() override;
 
+    /**
+     * Return whether the widget initialized properly. This could return false if Kiway is
+     * not available. If this returns false, no other methods should be called.
+     */
+    bool IsInitialized() const override { return m_preview != nullptr; }
+
     /**
      * Set the contents of the status label and display it.
      */