+ add further log messages at startup for better troubleshooting, fix possible crash, fix typos

src/Base/UnitsApi.cpp

@@ -80,16 +80,19 @@ UnitsApi::~UnitsApi()
 
 void UnitsApi::setSchema(UnitSystem s)
 {
-    if(UserPrefSystem){
+    if (UserPrefSystem) {
         UserPrefSystem->resetSchemaUnits(); // for schemas changed the Quantity constants
         delete UserPrefSystem;
+        UserPrefSystem = 0;
     }
     switch (s) {
         case SI1 : UserPrefSystem = new UnitsSchemaInternal(); break;
         case SI2 : UserPrefSystem = new UnitsSchemaMKS(); break;
         case Imperial1: UserPrefSystem = new UnitsSchemaImperial1(); break;
         case ImperialDecimal: UserPrefSystem = new UnitsSchemaImperialDecimal(); break;
+        default  : UserPrefSystem = new UnitsSchemaInternal(); s = SI1; break;
     }
+
     actSystem = s;
     UserPrefSystem->setSchemaUnits(); // if necesarry a unit schema can change the constants in Quantity (e.g. mi=1.8km rather then 1.6km).
 }