Fix :show for animation functions with empty cells

notebooks/tests/AnimationFunctionTestProB2.mch

+MACHINE AnimationFunctionTestProB2
+	DEFINITIONS
+		ANIMATION_FUNCTION0 == (1..2)*(1..2)*{0};
+		ANIMATION_FUNCTION1 == x;
+		ANIMATION_FUNCTION2 == {(0,0,"Overview")};
+		ANIMATION_IMG0 == "images/small_empty_w.gif";
+		ANIMATION_IMG1 == "images/small_wqueen_w.gif"
+	CONSTANTS x
+	PROPERTIES
+		x = {1|->2|->1, 2|->1|->1}
+END
\ No newline at end of file

notebooks/tests/show.ipynb

@@ -671,6 +671,112 @@
    "source": [
     ":show"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Empty cells are supported."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 22,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "Loaded machine: AnimationFunctionTestProB2"
+      ]
+     },
+     "execution_count": 22,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    ":load AnimationFunctionTestProB2.mch"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 23,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "Machine constants set up using operation 0: $setup_constants()"
+      ]
+     },
+     "execution_count": 23,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    ":constants"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 24,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "Machine initialised using operation 1: $initialise_machine()"
+      ]
+     },
+     "execution_count": 24,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    ":init"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 25,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/markdown": [
+       "<table style=\"font-family:monospace\"><tbody>\n",
+       "<tr>\n",
+       "<td style=\"padding:10px\">Overview</td>\n",
+       "<td style=\"padding:0px\"></td>\n",
+       "<td style=\"padding:0px\"></td>\n",
+       "</tr>\n",
+       "<tr>\n",
+       "<td style=\"padding:0px\"></td>\n",
+       "<td style=\"padding:0px\">![0](images/small_empty_w.gif)</td>\n",
+       "<td style=\"padding:0px\">![1](images/small_wqueen_w.gif)</td>\n",
+       "</tr>\n",
+       "<tr>\n",
+       "<td style=\"padding:0px\"></td>\n",
+       "<td style=\"padding:0px\">![1](images/small_wqueen_w.gif)</td>\n",
+       "<td style=\"padding:0px\">![0](images/small_empty_w.gif)</td>\n",
+       "</tr>\n",
+       "</tbody></table>"
+      ],
+      "text/plain": [
+       "<Animation function visualisation>"
+      ]
+     },
+     "execution_count": 25,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    ":show"
+   ]
   }
  ],
  "metadata": {

src/main/java/de/prob2/jupyter/commands/ShowCommand.java

@@ -99,7 +99,10 @@ public final class ShowCommand implements Command {
 			for (final Object entry : row) {
 				final int padding;
 				final String contents;
-				if (entry instanceof Integer) {
+				if (entry == null) {
+					padding = 0;
+					contents = "";
+				} else if (entry instanceof Integer) {
 					padding = imagePadding;
 					contents = String.format("![%d](%s)", entry, images.get(entry));
 				} else if (entry instanceof String) {