Fix/improve handling of two exceptions

BlueMapCommon/src/main/java/de/bluecolored/bluemap/common/plugin/Plugin.java

@@ -486,12 +486,16 @@ public class Plugin implements ServerEventListener {
 
     private void checkPausedByPlayerCountSoon() {
         // check is done a second later to make sure the player has actually joined/left and is no longer on the list
-        daemonTimer.schedule(new TimerTask() {
-            @Override
-            public void run() {
-                checkPausedByPlayerCount();
-            }
-        }, 1000);
+        try {
+            daemonTimer.schedule(new TimerTask() {
+                @Override
+                public void run() {
+                    checkPausedByPlayerCount();
+                }
+            }, 1000);
+        } catch (IllegalStateException ex) { // Timer is cancelled for some reason
+            Logger.global.logWarning("Timer is already cancelled, skipping player-limit checks!");
+        }
     }
 
     public boolean checkPausedByPlayerCount() {

BlueMapCore/src/main/java/de/bluecolored/bluemap/core/map/lowres/LowresTile.java

@@ -28,6 +28,10 @@ public class LowresTile {
         this.size = tileSize.add(1, 1); // add 1 for seamless edges
         this.texture = ImageIO.read(in);
 
+        if (this.texture == null) {
+            throw new IOException("No registered ImageReader is able to read the image-stream");
+        }
+
         if (this.texture.getWidth() != this.size.getX() || this.texture.getHeight() != this.size.getY() * 2) {
             throw new IOException("Size of tile does not match");
         }