eeschema: Load using root sheet if null

The root sheet is not set for pasting, in which case, we should utilize
the global root if needed.

Fixes #3956 | https://gitlab.com/kicad/code/kicad/issues/3956

eeschema/sch_legacy_plugin.cpp

@@ -754,6 +754,11 @@ void SCH_LEGACY_PLUGIN::LoadContent( LINE_READER& aReader, SCH_SCREEN* aScreen,
 {
     m_version = version;
 
+    // We cannot safely load content without a set root level.  If we haven't been given one,
+    // pick the default
+    if( m_rootSheet == nullptr )
+        m_rootSheet = g_RootSheet;
+
     while( aReader.ReadLine() )
     {
         char* line = aReader.Line();