Check whether flow is defined before rendering the preview button (#337)

frontend/src/lib/components/FlowBuilder.svelte

@@ -196,24 +196,23 @@
 					bind:schema={$flowStore.schema}
 				/>
 			{/if}
+			<Button
+				disabled={pathIsEmpty($flowStore.path)}
+				size="lg"
+				pill
+				on:click={() => (previewOpen = !previewOpen)}
+				class={`blue-button fixed bottom-10 right-10 ${previewOpen ? 'hidden' : ''}`}
+			>
+				Preview flow
+				{pathIsEmpty($flowStore.path) ? '(pick a name first!)' : ''}
+
+				<Icon data={faPlay} class="ml-2" />
+			</Button>
 		{:else}
 			<p>Loading</p>
 		{/if}
 	</div>
 
-	<Button
-		disabled={pathIsEmpty($flowStore.path)}
-		size="lg"
-		pill
-		on:click={() => (previewOpen = !previewOpen)}
-		class={`blue-button fixed bottom-10 right-10 ${previewOpen ? 'hidden' : ''}`}
-	>
-		Preview flow
-		{pathIsEmpty($flowStore.path) ? '(pick a name first!)' : ''}
-
-		<Icon data={faPlay} class="ml-2" />
-	</Button>
-
 	<div class={`relative h-screen w-1/3 ${previewOpen ? '' : 'hidden'}`}>
 		<div class="absolute top-0 h-full">
 			<div class="fixed border-l-2 right-0 h-screen w-1/3">