IRewinder - dress up the api a bit, fix a potential NRE

src/BizHawk.Client.Common/rewind/Rewinder.cs

@@ -12,10 +12,12 @@ namespace BizHawk.Client.Common
 		int RewindFrequency { get; }
 
 		bool Active { get; }
-		bool Suspend { get; set; }
 
 		void Capture(int frame);
 		bool Rewind(int frames);
+
+		void Suspend();
+		void Resume();
 	}
 
 	public class Rewinder : IRewinder
@@ -33,11 +35,11 @@ namespace BizHawk.Client.Common
 		private bool _lastRewindLoadedState;
 		private byte[] _deltaBuffer = new byte[0];
 
-		public bool Active => RewindEnabled && !Suspend;
+		public bool Active => RewindEnabled && !_suspend;
 
 		private bool RewindEnabled { get; }
 
-		public bool Suspend { get; set; }
+		private bool _suspend;
 
 		public float FullnessRatio => _rewindBuffer?.FullnessRatio ?? 0;
 
@@ -79,6 +81,16 @@ namespace BizHawk.Client.Common
 			}
 		}
 
+		public void Suspend()
+		{
+			_suspend = true;
+		}
+
+		public void Resume()
+		{
+			_suspend = false;
+		}
+
 		private void Clear()
 		{
 			_rewindBuffer?.Clear();

src/BizHawk.Client.EmuHawk/MainForm.cs

@@ -4009,12 +4009,22 @@ namespace BizHawk.Client.EmuHawk
 
 		public void EnableRewind(bool enabled)
 		{
-			if (Rewinder == null && Emulator.HasSavestates())
+			if (!Emulator.HasSavestates())
 			{
-				Rewinder = new Rewinder(Emulator.AsStatable(), Config.Rewind);
+				return;
+			}
+
+			Rewinder ??= new Rewinder(Emulator.AsStatable(), Config.Rewind);
+
+			if (enabled)
+			{
+				Rewinder.Resume();
+			}
+			else
+			{
+				Rewinder.Suspend();
 			}
 
-			Rewinder.Suspend = !enabled;
 			AddOnScreenMessage($"Rewind {(enabled ? "enabled" : "suspended")}");
 		}