Add back null check/exception

osu.Framework/Timing/DecoupleableInterpolatingFramedClock.cs

···
             get => Source?.Rate ?? 1;
             set
             {
-                if (adjustableSource != null)
-                    adjustableSource.Rate = value;
+                if (adjustableSource == null)
+                    throw new NotSupportedException("Source is not adjustable.");
+
+                adjustableSource.Rate = value;
             }
         }