Fix crash when restarting preview threads

Wait at most 5s for previous runner to stop so ui thread doesn't get locked if there was an error
Handle cases where TextureEntries are returned null due to thread interruption

src/org/jmc/BlockInfo.java

@@ -136,6 +136,9 @@ public Color getPreviewColor(BlockData blockData, NamespaceID biome)
 		} else {
 			te = Registries.getTexture(mtlNames[0]);
 		}
+		if (te == null) {
+			return Color.MAGENTA;
+		}
 		try {
 			return te.getAverageColour();
 		} catch (IOException e) {

src/org/jmc/gui/MainPanel.java

@@ -716,7 +716,7 @@ void stopPreviewLoader() {
 		if (chunk_loader_thread != null) {
 			chunk_loader_thread.interrupt();
 			try {
-				chunk_loader_thread.join();
+				chunk_loader_thread.join(5000);
 			} catch (InterruptedException e) {
 				Log.error("Interrupted waiting for preview chunk loader to stop!", e);
 				Thread.currentThread().interrupt();

src/org/jmc/gui/ViewChunkLoaderRunner.java

@@ -221,7 +221,7 @@ public void run() {
 				t.interrupt();
 				try {
 					t.join();
-				} catch (InterruptedException e) {}
+				} catch (InterruptedException ignored) {}
 			}
 			imagerThreads.clear();
 		}

src/org/jmc/models/Banner.java

@@ -241,6 +241,8 @@ private boolean generateBannerImage(NamespaceID materialImageName, ArrayList<Ban
             Log.error("Cant read banner base image!", e, showReadErrorPopup());
             return false;
         }
+		if (backgroundImage == null) return false;
+
         // todo - do something with the basecolor here...
         // Log.info(" - Base Color: " + baseColorIndex);
 
@@ -257,6 +259,7 @@ private boolean generateBannerImage(NamespaceID materialImageName, ArrayList<Ban
             BufferedImage patternSource;
             try {
             	TextureEntry te = Registries.getTexture(new NamespaceID("jmc2obj", "banner/pattern_" + bp.getPattern()));
+				if (te == null) return false;
             	patternSource = te.getImage();
             } catch (IOException e) {
             	Log.error("Cant read banner pattern " + bp.getPattern(), e, showReadErrorPopup());
@@ -306,6 +309,7 @@ private boolean generateBannerImage(NamespaceID materialImageName, ArrayList<Ban
         }
 
         TextureEntry texEntry = Registries.getTexture(materialImageName);
+		if (texEntry == null) return false;
         texEntry.setImage(combined);
         return true;
     }

src/org/jmc/models/Head.java

@@ -220,6 +220,9 @@ private static boolean exportSkullOwnerTexture(TAG_Compound skullOwnerTag, Names
 				texture = expandedTex;
 			}
 			TextureEntry texEntry = Registries.getTexture(texId);
+			if (texEntry == null) {
+				return false;
+			}
 			texEntry.setImage(texture);
 			return true;
 		} catch (IOException e) {

src/org/jmc/registry/Registries.java

@@ -104,6 +104,7 @@ public static ModelEntry getModel(NamespaceID id) {
 		return models.get(id);
 	}
 
+	@CheckForNull
 	public static TextureEntry getTexture(NamespaceID id) {
 		return textures.get(id);
 	}